We examined effects of newly discovered marine coral prostanoids, clavulones, isolated from the Japanese stolonifer Clavularia viridis, on the spontaneous beating rate of cultured myocardial cells from fetal mouse hearts. Clavulone caused positive chronotropic action at 2-5 min after addition of clavulone (0.45 microM) to the reaction media of cardiac cells. This effect induced by clavulone was clearly different from the positive inotropic effects of ouabain (10 microM) and Bay K 8644 (0.1 microM) as judged by photoelectric recordings of beating. These results suggest a new biological action of clavulone that has positive chronotropic action on the cultured mouse myocyte preparation.