1. Membrane potential and tension were measured simultaneously in ring segments of main coronary artery of guinea‐pigs. The synthetic thromboxane A2 analogue U46619 depolarized the tissues from ‐58 +/‐ 2 to ‐40 +/‐ 1 mV and increased tension by 12 +/‐ 1 mN mm‐1. Nitric oxide (NO) and Iloprost, the stable analogue of prostacyclin, evoked hyperpolarization and relaxation. 2. The concentration of NO required to evoke half‐maximal hyperpolarization (EC50 of 2 x 10(‐5) M) was 40‐fold higher than that which was required to induce relaxation (EC50 of 5 x 10(‐7) M). The EC50 for Iloprost‐induced hyperpolarization (3 x 10(‐8) M) was similar to that for relaxation (4 x 10(‐8) M). 3. Glibenclamide (10(‐6) M) abolished the hyperpolarization in response to both NO and Iloprost but was without effect on the amplitudes of the relaxations over the complete concentration‐response curves. 4. Acetylcholine evoked concentration‐dependent hyperpolarization and relaxation in the presence of N omega‐nitro‐L‐arginine methyl ester (NAME; 10(‐5) M) and indomethacin (10(‐6) M), and these responses were attributed to endothelium‐derived hyperpolarizing factor (EDHF). The hyperpolarization produced by EDHF always preceded relaxation, and relaxation never occurred at concentrations of acetylcholine that were insufficient to evoke hyperpolarization. 5. The concentration‐hyperpolarization and concentration‐relaxation curves in response to acetylcholine were not affected by glibenclamide or barium (1‐3 mM) but were shifted to the right 4‐ and 5‐fold, respectively, by 1 mM tetraethylammonium. The hyperpolarization and relaxation evoked by acetylcholine were also reduced in a parallel manner when the potassium concentration in the superfusate was increased. 6. Hyperpolarizing current steps, applied to spiral strips of coronary artery denuded of endothelium and depolarized and constricted with U46619, caused relaxation. The relationship between hyperpolarization and relaxation evoked electronically was similar to that which was due to EDHF in intact tissues stimulated with acetylcholine. 7. It is concluded that the ability of NO or Iloprost to relax guinea‐pig coronary artery does not depend upon hyperpolarization of the smooth muscle. In contrast, hyperpolarization is likely to play a major, if not the only, role in the relaxation in response to EDHF in this tissue.