In rats, injection of chlorpromazin hydrochloride (CPZ), a calmodulin antagonist, to each uterine horn had a significant (87%) anti-implantation effect and 100% antifertility effect. Both of these rates were zero in the saline control group. Polyacrylamide gel electrophoresis analysis of the uterine fluid showed a decrease in the protein content and certain changes in its chemical composition, in particular, a marked reduction in the number of bands of estradiol-induced 70 kD proteins in the experimental group. No significant difference was found in the ovarian weight and the serum estradiol (E2) and progesterone (P) levels between the two groups. These results suggest that calmodulin influences the process of implantation and this effect is not directly related to the serum concentrations of E2 and P. It is believed that CPZ probably exerted its anti-implantation effect by a direct action on the uterus.