In guinea pig ventricular myocytes the kappa-opioid agonist trans(+/-)-3,3-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]- benzeneacetamide methanesulfonate salt (U50488H) inhibited L-type calcium currents (ICa) as measured by the patch clamp technique in a dose-dependent fashion. The basal, as well as the isoprenaline and 8-Br-cAMP-stimulated, ICa were found to be reduced by U50488H. The inhibition was reversible and was not prevented on preincubation with pertussis toxin or the receptor antagonists naloxone and naltrexone. Naltrexone alone also caused an inhibition of ICa. Leucine enkephaline, a peptide opiate agonist, had no effect on ICa. U50488H did not alter the current/voltage relationship of the calcium current. The inhibition was independent of the cytosolic calcium concentration because it was also observed in the presence of 10 mM BAPTA in the pipette. If the compound was applied intracellularly via a perfused patch pipette there was no inhibition of ICa. The calcium current stimulated by the dihydropyridine calcium agonist Bay K 8644 was also reduced by U50488H. Conversely the inhibition of ICa by U50488H could be antagonized by Bay K 8644. In conclusion, these results demonstrate that binding to specific membrane receptors is not involved in the inhibition of L-type calcium current by U50488H and other nonpeptide opioid agonists. A direct interaction with the channel molecule at the exterior of the cell is probably involved.