1. In rabbit basilar arteries, melatonin (10(-5)-10(-3) M) and nifedipine (10(-10)-10(-8) M) inhibited the responses to KCl and 5-hydroxytryptamine. 2. Melatonin or nifedipine inhibited the Ca2+-responses in a Ca2+-free medium containing KCl (40 mM) or 5-HT (10(-5) M). 3. A combined treatment with nifedipine and melatonin caused no further inhibition of the Ca2+ response in the presence of K+ as compared to a single treatment with melatonin. 4. However, the combined treatment caused a greater inhibition of the Ca2+-response in the presence of 5-HT than the single treatment with either agent. 5. These results suggest that melatonin and nifedipine inhibit the same Ca2+ channels activated by KCl. Further, 5-HT-activated Ca2+ channels which are inhibited by melatonin may be different from nifedipine-sensitive Ca2+ channels activated by 5-HT.