The capacity of two maize opaque endosperm mutants (o1 and o2) and two floury (fl1 and fl2) to accumulate lysine in the seed in relation to their wild type counterparts Oh43+ was examined. The highest total lysine content was 3.78% in the o2 mutant and the lowest 1.87% in fl1, as compared with the wild type (1.49%). For soluble lysine, o2 exhibited over a 700% increase, whilst for fl3 a 28% decrease was encountered, as compared with the wild type. In order to understand the mechanisms causing these large variations in both total and soluble lysine content, a quantitative and qualitative study of the N constituents of the endosperm has been carried out and data obtained for the total protein, nonprotein N, soluble amino acids, albumins/globulins, zeins and glutelins present in the seed of the mutants. Following two-dimensional PAGE separation, a total of 35 different forms of zein polypeptides were detected and considerable differences were noted between the five different lines. In addition, two enzymes of the aspartate biosynthetic pathway, aspartate kinase and homoserine dehydrogenase were analyzed with respect to feedback inhibition by lysine and threonine. The activities of the enzymes lysine 2-oxoglutate reductase and saccharopine dehydrogenase, both involved in lysine degradation in the maize endosperm were also determined and shown to be reduced several fold with the introduction of the o2, fl1 and fl2 mutations in the Oh43+ inbred line, whereas wild-type activity levels were verified in the Oh43o1 mutant.