BACKGROUND: Effects of fentanyl, sufentanil, meperidine, and morphine on the spontaneous contractility of isolated human pregnant uterine muscle strips were determined. METHODS: Uterine specimens were obtained from normal full-term parturients undergoing elective lower-segment cesarean section. Longitudinal muscle strips were made and mounted individually and vertically in tissue chambers to record their isometric tension. After establishment of rhythmic contractions in the buffer solution, opioid concentration-response curves were constructed. The responses to opioids were repeated in the presence of opioid receptor blocker, nitric oxide synthase inhibitor, beta-adrenoceptor blocker, or cyclo-oxygenase inhibitor. RESULTS: Fentanyl and meperidine caused concentration-dependent decreases of the uterine contractility, their IC50 (concentration which causes 50% inhibition of the amplitude of spontaneous contractions) being 6.8 x 10(-6) M and 2.2 x 10(-3) M, respectively. On the contrary, sufentanil and morphine were without significant effects on the contractility. Pretreatment with either naloxone, N(G)-nitro-L-arginine methyl ester, atenolol, or indomethacin did not affect the uterine responses to opioids. CONCLUSIONS: These results demonstrate that fentanyl and meperidine may have direct inhibitory effects on the contractility of the human uterus. However, the opioid concentrations needed to significantly reduce the uterine contractility were at a supraclinical range.