Maturing seeds of six peanut cultivars (Arachis hypogaea L.) varying in protein content at maturity showed differences in rate of change of dry weight, crude protein, and free and total amino acids. Seeds of the high-protein cultivars increased in dry weight and deposited protein at a more rapid rate between immature and low intermediate stages of maturation than did those of the low- and intermediate-protein cultivars. Free amino acid content in seeds classified as low-intermediate maturity from all cultivars was significantly less than was that of those at the immature stage. The greatest change was in seed of the high-protein group. The rate of change of content of selected free amino acids among seed was different for the three groups of cultivars. Similar observations were noted as the content of select total amino acids increased in maturing seeds. Variations in quantities of free amino acids in immature seeds and differences in the rate at which they are incorporated into proteins of seeds from various cultivars suggest that there is genetic variability in the mechanism for synthesis of selected proteins. These differences also indicate the potential for the development of peanut cultivars having seed with nutritionally desirable protein.