In normal cyclic female rats, an injection of 16 mg CuSO4/kg induces pseudopregnancy (as determined by uterine traumatization) when given on the day of oestrus, or on the following day. Pentobarbital anaesthesia did not inhibit this effect of CuSO4. Pseudopregnancy, as determined by inhibition of the induction of vaginal cornification by exogenous oestrogen, was also induced by CuSO4 in anovulatory females, provided the injection was given from 6 hr before, to 24 hr after injection of human chorionic gonadotrophin. The latter was necessary since CuSO4 did not induce ovulation in forty experimentally produced anovulatory rats. Cupric sulphate injected on the day of vaginal pro-oestrus in normal females did not alter the incidence of spontaneous ovulation nor the number of ova shed. These data suggest an effect of copper ion in the rat on luteal regulatory mechanisms rather than on ovulatory mechanisms; the site of action cannot, however, be specified.