The possible inotropic effects of all three classes of endogenous opioids were tested alone or in combination with noradrenaline, adrenaline, or carbachol on electrically stimulated atria isolated from male Sprague-Dawley rats. Noradrenaline (6.0 and 12 microM) and adrenaline (4.0 and 8.0 microM) injections caused marked but transient (5 min) dose-related increases in atrial tension compared with preinjection control values, whereas carbachol (0.14 and 1.4 microM) caused a more potent and prolonged (over 15 min) dose-related decrease in atrial tension development. Adrenal enkephalins (0.3-4.0 microM) of methionine enkephalin, leucine enkephalin, Met-enkephalin-Arg6-Phe7, and Met-enkephalin-Arg6-Gly7-Leu8, beta-endorphin (0.2-2.0 microM), or dynorphin A(1-13) (0.2-2.0 microM) did not change atrial tension for a 15-min postadministration test period. In addition, these opioids did not affect the positive inotropic effects of noradrenaline (12 microM) or adrenaline (8.0 microM) or the negative inotropic actions of carbachol (1.4 microM) when the same doses of noradrenaline, adrenaline, or carbachol were given alone. These data indicate that endogenous opioids given in micromolar concentrations tested did not affect atrial tension development of electrically stimulated rat atria. Comparing these data with those of past literature, it is suggested that circulating endogenous opioids probably do not have any direct effects on the rat myocardium to affect myocardial contractility.