Opioid peptides exert ino- and chronotropic effects on the heart. Paraopioid dermorphine (Y-dA-F-G-Y-P-S) was earlier shown to act as negative inotropic agent in isolated frog's hearts. In this study, the same effect was investigated in cultured myocardiac cells of the newborn rats. Dermorphine (10(-4)-10(-9) M) dose-dependently attenuated the contraction amplitude with the latency of 5-10 min. The peak of the response appeared in 30-45 min after application of dermorphine. The mu-opiate-receptor antagonist naloxone (10(-5) g/ml) blocked this effect. Dermorphine seems to exert its negative inotropic action through the opiate receptors, presumably of the mu-subtype.