1. Multiple effects of the imidazole compound SK&F 96365 have been evaluated on endothelial cells from human umbilical vein using a combined patch clamp and Ca(2+)-microfluorimetric technique (Fura-2). 2. At concentrations of 100 mumol/l or higher of SK&F 96365, the block of the receptor-mediated Ca2+ entry overlaps with the activation of another Ca(2+)-entry mechanism, which is associated with a non selective cationic current. 3. This rise in [Ca2+]i depends on the extracellular Ca(2+)-concentration, and the entry pathway is in contrast with the receptor-mediated Ca(2+)-entry pathway permeable to Ni2+, as shown by quenching of the Fura-2 fluorescence signal. 4. The concentration of SK&F 96365 for half maximal increase in [Ca2+]i was 141 +/- 19 mumol/l (n = 16). 5. SK&F 96365 activated a current that reversed at +11.8 +/- 2.1 mV (n = 21) when measured using nystatin-perforated patches with either Cs+ or K+ in the pipette and 140 Na+, 1.5 Ca2+ in the bath (chloride equilibrium potential ECl = -36 mV). 6. SK&F 96365 (200 mumol/l) blocked an inwardly rectifying K+ current in endothelial cells independently of [Ca2+]i. This block caused depolarization of the endothelial cells from -55.3 +/- 2.57 mV (n = 33) to -10 +/- 5.5 mV (n = 6). This block was concentration-dependent, half maximal block occurred at a concentration of about 40 mumol/l SK&F 96365. 7. In cells which showed an outwardly rectifying current, this outward component was also completely blocked by 200 mumol/l SK&F 96365. 8. It is concluded that SK&F 96365 reversibly activates a non-selective cation channel at concentrations higher than 100 mumol/l, but also blocks K+ currents in endothelial cells independently of [Ca2+]i. These multiple effects overlap with the proposed block of receptor-mediated Ca2+ entry. The block of K(+)-channels may in unclamped cells reduce the driving force for Ca2+, and thereby interfere with the Ca(2+)-influx.