An investigation on the effect of RU486 on isolated ovicluct smooth muscle contraction of pseudopregant rabbits was carried out by using the mechanical-electrical transducer of Galson recorder. The results showed that: (1) RU486 acted directly on the oviduct smooth muscle by increasing contractile frequency, without changing tension and amplitude. This effect is similar to that found in the in vivo experiment. (2) RU486 partially inhibited Ca(2+)-induced smooth muscle contraction and also showed significant synergistic effect of Verapamil and antagonistic effect of NE on muscle contractility. However, RU486 is unable to exert any effect on Forskolin-stimulated contraction. Thus it appears that the RU486 effect on oviduct smooth muscle contraction is a result of affecting intracellular free calium, due possibly to interference of Ca2+ influx and/or endoplasmic reticulum Ca2+ release and Ca(2+)-Ip3 transducing mechanism.