Effects of intraventricular injections of 6-hydroxydopamine (6-OHDA; 200 μg/20 μl vehicle) on anterior pituitary cell proliferation in rats have been investigated by means of the colchicine metaphas-arrest techniques. In those groups of animals receiving 6-OHDA alone or 6-OHDA plus desmethylimipramine (DMI, 25 mg/kg body weight i.p.), where the mean mitotic activity rates (MMARs) were initially low at 48 hours, an increase of MMARs was observed at 96 and 144 hours after the drug injections. At 144 hours after drug administration, the MMARs values in the 6-OHDA-injected group and in the 6-OHDA+DMI-treated group were significantly higher than those in the control groups. This increase of MMARs resulted from the enhancement of acidophilic and chromophobe cell proliferation. The low MMARs at 48 hours after 6-OHDA injection are probably a result of dopamine release from damaged nerve endings; the enhancement of MMARs (particularly evident in the 6-OHDA+DMI-treated group) at 96 and 144 hours after drug adminstriation is presumably related to a deficiency of dopaminergic control of anterior pituitary cell proliferation.