The developmental onset of three homocysteine metabolizing enzymes in the rat conceptus was investigated. Cystathionine-beta-synthase and methionine synthase were assayed from day 10 to day 20 of gestation in decidual and placental tissue, from day 10 to day 12 of gestation in embryonic tissue, from day 14 to day 20 of gestation in fetal liver and from day 14 to day 20 of gestation in fetal tissue without liver. On each day, material was obtained from at least four conceptuses from two dams. S-adenosylhomocysteine hydrolase was assayed in neurulating conceptuses in decidual tissue, parietal yolksac plus ectoplacental cone, visceral yolksac plus amnion and embryo proper. Conceptuses were pooled from seven (day 9.5 of gestation) or three (days 10.5 and 11.5 of gestation) dams. In embryonic and fetal tissue cystathionine-beta-synthase first occurred in fetal liver. During the organogenic phase it was present only in decidual tissue. Methionine synthase was present in all tissues from the first gestational day investigated and S-adenosylhomocysteine hydrolase was present in all tissues throughout the neurulating period. Our results indicate that the homocysteine-methionine cycle, which is crucial to transmethylation reactions, is functional during the neurulating period in embryonic tissue. Owing to the absence of cystathionine-beta-synthase at this stage of development in embryonic tissue, the homocysteinyl moiety is conserved in the homocysteine-methionine cycle.