A bimodal change in yield and microsomal protein content of rabbit mammary gland was observed with the progress of pregnancy and lactation. The initial stimulus took place on day 22 of pregnancy and the second during early lactation. Palmitoyl-CoA:monopalmitoyl-sn-glycerol 3-phosphate palmitoyltransferase activity was monitored concurrently. This enzyme in rabbit mammary microsomes is composed of two isoenzymic species that differ with respect to the physical nature of the substrates with which each interacts. The activities of the two isoenzymes were recorded at progressive stages of pregnancy, lactation, and involution to determine if a regulatory role could be assigned to either or both species. Although the patterns were indefinite, total transacylase activity did increase over this period, i.e., the specific activity of LPAT-alpha was 12 and 24 nmoles/mg protein per min in pregnancy and lactation, respectively, while that of LPAT-beta rose from zero to 90 nmoles/mg protein per min over the same period. The time of harvesting in relation to the interval between nursing periods is discussed in the light of these results.