The effects of endothelin-1 (ET-1) on surface membrane Ca2+ channels were studied on cultured human embryonal vascular smooth muscle cells (VSMC) and on isolated rat aorta using photoaffinity labelling with DHP Ca2+ channel antagonist (-)-[3H]-azidopine (AZI). The AZl-labelled saturable population of sites on VSMC with Bmax = 1.59±0.10 pmol/mg of protein and Kd = 5.40±1.70 nmol/1; and 1.32±0.11 pmol/mg w.w. and Kd = 1.09±0.20 nmol/1 in isolated rings of the rat aorta. Preincubation with ET-1 (0.1, 1.0 and 10 nmol/1) increased (in a concentration-dependent manner) the total number of sites specifically photolabelled on VSMC. The number of sites labelled with AZI on ET-1 preincubated VSMC increased markedly when divalent cations (Ca2+ or Mg2+ in other experiments) were present in the incubation medium. Specific photolabclling also significantly increased in VSMC pretreated with intrinsically photoreactive nifedipine. A protein kinase C inhibitor staurosporine, added to the incubation medium, significantly reduced the enhanced specific photolabelling after ET-1. The increase in specific photolabelling after ET-1 preincubation (+ 197±46 %; P<0.05) was also observed in rings of the rat aorta and it was significantly reduced after prcincubation with S-(+)-niguldipine.