The immediate effect of HCG upon plasma testosterone levels in the bull.

No effect of HCG upon the plasma testosterone level in 16 bulls was seen the first 5 or 15 min after injection. In 8 bulls receiving 750 or 1500 i.u. HCG by intravenous injection a lag time of 30 min occurred before plasma testosterone response could be measured. The high plateau level of plasma testosterone concentration was reached approximately 1 h after injection. Following intramuscular injection of 750 i.u. or 1500 i.u. HCG a lag time of 45–60 min was observed for the testicular response measured as elevated plasma testosterone level. The high plateau levels of plasma testosterone in these bulls were reached approximately 1½ h post injection.