It is known that KATP channel openers inhibit the release and refilling of Ca2+ from intracellular stores. The present study was designed to test the effects of levcromakalim in human umbilical artery (HUA) rings stimulated by serotonin (5-HT) and KCl in Ca-free medium. Umbilical cords were obtained at vaginal or cesarean deliveries from healthy, term pregnancies. After the isolation, HUA rings were placed in organ baths in solution with indomethacin (10-5 M) and NG-nitro-L-arginine methyl ester (L-NAME) (10-3 M) at 37°C and aerated with 95% O2 and 5% CO2 for the measurement of isometric force. In Ca-free solution with Ethylene glycol-bis (β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid (EGTA) (2 mM) the contractions produced by 5-HT (10-6 M) and KCl (40 mM) decreased significantly. Afterwards, HUA rings were treated with 5-HT and KCl in repeated manner in Ca-free medium. In contrast to KCl, 5-HT induced contractions reduced in each application, progressively. Levcromakalim (10-4 M) abolished the contractions elicited by 5-HT. On the other hand, levcromakalim had a little but significant inhibitory effect on KCl induced contraction in Ca-free medium. These results suggest that Ca2+ is not the only transduction pathway in KCl produced contractions of HUA smooth muscle cells. © 2002 Elsevier Science Inc. All rights reserved.