The possibility that biogenic amines affect ovarian development in the red swamp crayfish,Procambarus clarkii, was investigated. Females were administered 15 μg/g body weight (bw) of norepinephrine, dopamine, 5-hydroxytryptamine (5-HT) or octopamine on days 1, 5 and 10 and were sacrificed on day 15. Crayfish given 5-HT showed significant increases in ovarian index (30.5%) and oocyte size (34.0%) over the concurrent controls, while norepinephrine, dopamine and octopamine did not significantly affect either the ovarian index or oocyte size. Significantly more labeling by14C-leucine of ovarian proteins was found in ovaries of crayfish that were injected with 5-HTin vivo, but when ovarian lobes from crayfish that had not been injected with 5-HT were incubatedin vitrowith 5-HT added to the incubation medium, no significant change in the level of incorporation of14C-leucine into ovarian proteins occurred.