Abstract Our lab has recently shown that T-lymphocyte function ex vivo from glycerol-3-phosphate acyltransferase-1 (GPAT-1), first enzyme in de novo glycerophospholipid biosynthesis, knockout mice is similar to aged T-lymphocytes. In order to determine the mechanism(s) for this observation, we examined thymic development in GPAT-1 KO mice. Thymi were weighed then thymocytes isolated for determination of proliferation and thymic subset proportions. We found that the thymi from GPAT-1 KO mice were 30% smaller than the wild type controls and thymocyte proliferation was reduced. Furthermore, the thymic single positive CD4 and double negative subset proportions were increased while the double positive population was decerased. The changes in thymic weight and thymocyte subsets are most likely not linked to altered fetal thymic development since thymic weights from 3 week old mice were identical to that seen in wild type controls. Since T-lymphocyte GPAT-1 activity is reduced in aged T-lymphocytes these data give insight into a novel mechanism by which aging may alter thymic development and subsequent peripheral T-lymphocyte function. This work was supported by NIH AG20651 (CAJ).