Serine palmitoyltransferase (EC 2.3.1.50) initiates the biosynthesis of sphingolipids. Its activity is induced in the aortas of rabbits fed a Purina lab chow supplemented with 2% cholesterol (Williams, R. D., D. S. Sgoutas, and G. S. Zaatari. 1986. J. Lipid Res. 27: 763-770). Induction occurs during atherogenesis in parallel with increased arterial sphingomyelin concentrations. In this study, L-cycloserine was shown to be a potent inhibitor of serine palmitoyltransferase in aortas from New Zealand White rabbits. Activity was reduced in vitro by 50% using 5 microM L-cycloserine with 50 micrograms of microsomal protein. To assess in vivo inhibition, L-cycloserine was administered by intraperitoneal injection to rabbits maintained on either a standard Purina laboratory chow or one supplemented with 2% cholesterol. Serine palmitoyltransferase activity was inhibited by 76% in the aortas of rabbits on the standard chow 4 hr after a single 25 mg/kg body weight dose and 52% after a 10 mg/kg dose. Activity was reduced by 36% in animals on the standard chow and by 37% in the cholesterol-fed group after 1 week of daily doses. These experiments demonstrate that L-cycloserine inhibits serine palmitoyltransferase in aorta, and thus may be used to reduce sphingomyelin concentrations during experimental atherogenesis.