Pregnant mare serum gonadotrophin given intravenously to immature rats caused a maximal (x 70) increase in ornithine decarboxylase activity (ODC) at 3 h; enzyme activity declined to about ten times the control levels by 9 h and a second rise began after about 20 h. Anti-PMSG given 30 min after PMSG reduced the peak response by 70%. Actinomycin D, or cycloheximide, completely prevented an increase in ODC when given with PMSG, but only cycloheximide lowered the enzyme activity when given 18 h later. Ovine FSH plus LH also produced a peak in ODC at 3 h but the activity decreased quickly and by 9 h it was at the control level. Secretion of endogenous FSH and LH, induced by hourly injections of LH releasing hormone (LH-RH) increased ODC to the same extent as did the exogenous hormones; ODC was still higher than in the controls 4 h after the last dose of LH-RH. Increased endogenous levels of FSH and LH did not consistently raise ovarian cyclic AMP content and the increases found were much less than those obtained after injection of PMSG or FSH+LH. The results indicate that increased ODC is induced and maintained by the continual presence of gonadotrophin. The dependence of increased ODC upon increased cyclic AMP cannot be unequivocally determined because of important differences in the timing of the responses and the difficulty in determining biologically significant changes in cyclic AMP.