Maize (Zea mays L.), being a major cereal crop, is used as both food and feed worldwide however, maize proteins are of poor nutritional quality due to deficiency of essential amino acids; lysine and tryptophan. Quality protein maize (QPM) breeding involves the combined use of the opaque-2 (o2) gene and the genetic modifiers of the o2 locus to develop cultivars with elevated levels of lysine and tryptophan. In the present study, four white QPM inbred lines were crossed in full diallel fashion in Summer 2006. Parental inbreds along with their 12 F1 hybrids were field evaluated at two locations of Khyber Pakhtunkhwa, Agricultural University Peshawar (AUP) and Cereal Crops Research Institute (CCRI), Pirsabak, Nowshera in Summer 2007. A randomized complete block design with three replications was used at each location. All the inputs and cultural practices were kept uniform at both test locations. Data was recorded to asses the gene action, general (GCA) and specific (SCA) combining ability and heterotic effects for morpho-physiological, yield, protein quantity and protein quality contributing traits. Highly significant differences for all the traits under study were found among genotypes (inbred lines and F1 hybrids) at both test locations. Genotype×location interaction effects were also significant for most of the morpho-physiological, yield and protein content and quality traits measured on four S5 white QPM inbred lines and their twelve F1 hybrid combinations. Genetic components of additive and dominance variation were significant at both test locations for most of the yield and protein quality traits. However, over dominant type of gene action was comparatively more pronounced than the additive component of variation for grain yield and yield contributing traits at both locations. At CCRI, preponderance of non-additive component of variation for lysine and tryptophan contents was observed while additive component for these traits were comparatively more prominent at AUP. Reciprocal effects at AUP influenced dominant and additive components of variation for ear height, biological yield, stover yield, 100-grain weight, ear weight, kernel weight ear-1, grain lysine and tryptophane content, and lysine and tryptophane concentrations in protein while at CCRI, kernel rows ear-1, grain lysine and tryptophan contents, and lysine and tryptophan concentrations in protein were influenced. Mean squares due to GCA and SCA were highly significant for all the important yield and quality traits investigated at both locations. The variance ratio of GCA to SCA revealed preponderance of non-additive gene action for protein quality, grain yield and its contributing traits at both test locations. The Inbred lines NCIQW13 and NCIQW3 performed well in general combinations for grain yield and lysine content at both test locations. The desirable SCA effects for grain yield at both locations were recorded for NC1QW5×NCIQW13. The best specific combinations for tryptophan and grain protein content were NC1QW1×NCIQW5 and NC1QW3×NCIQW5 respectively by expressing desirable SCA effects for grain protein content. Significant heterotic effects for 100-grain weight were recorded for cross combination NC1QW13×NCIQW1 (24.34 and 17.24%) with maximum value for 100-grain (34.7 and 31.2g) at both locations, respectively. Maximum grain yield at AUP was expressed by NC1QW5×NCIQW13 (5758 kg ha-1) while its reciprocal NC1QW1×NC1QW13 with 6908 kg ha-1 was best among the hybrids at CCRI. The same combination expressed comparatively desired grain lysine (0.385 and 0.380 g/100 g sample) and tryptophan (0.0865 and 0.0895 g/100 g sample) contents at AUP and CCRI, respectively. Mean performance of inbred lines NC1QW3 and NC1QW5 was comparatively better in general combinations for yield and protein quality traits and recommended for future breeding programs. Comparatively better combinations on the basis of average performance regarding yield and protein quality traits were NC1QW5×NCIQW13 and NC1QW1×NCIQW5. These hybrids may be useful transgressive segregants in subsequent generations and the inbred lines be exploited in future breeding program for yield and protein quality attributes. Based on different inheritance patterns for different traits it was inferred that over all performance of the inbreds and their all-possible cross combinations might be more useful while selecting for yield and quality enhancement. On the basis of these results, it is suggested that combinations with desirable values for protein and protein quality attributes across locations will be the cross combinations between lines with the highest GCA values for these parameters.