During the first 2/3 of gestation, the concentrations of UDPG, ATP, ADP, and Mg++ in human fetal liver remain constant, whereas the concentration of Pi decreases 2 fold and the G 6 P and AMP concentrations increase. Incubation of human fetal liver explants with glucagon or insulin did not alter the concentrations of any of these intermediates. ATP, ADP, and Pi are inhibitors of human fetal liver glycogen synthase D form activity, while G 6 P and AMP and Mg++ are stimulators. Ca++ at concentrations of less than 0.1 mM was found to stimulate glycogen synthase D activity. This effect of Ca++ was also observed in 'physiologic' mixtures containing UDPG, G 6 P, ATP, ADP, AMP, Pi, and Mg++ at concentrations found either in liver in utero or in explants. 45Ca++ efflux from perifused (rat) fetal liver explants was stimulated by glucagon. These data provide a picture of the metabolite regulation of human fetal liver glycogen synthase activity in which the D form may largely control glycogen synthesis in utero and hormonal effects on glycogen synthase may be induced by effects of Ca++ on the D form.