The purpose of this study was to investigate the effect of prolonged (14 days) intragastric administration of Mn2+ (0.25 mmol/kg daily) on Ca2+ exchange and contractility of rat ventricular myocardium. Left-ventricular pressure and its first derivative (dP/dt) were recorded by means of a balloon catheter inserted via the left atrium into the left ventricle of the rat heart perfused by Langendorff method. Ca2+ exchange in the stimulated and rested ventricular myocardium was investigated with the aid of 45Ca under the conditions of complete equilibration of preparations with a solution containing 45Ca2+. The "cellular" 45Ca2+ content was calculated by subtraction of 45Ca2+ dissolved in the free water of extracellular space from the total tissue 45Ca2+ content. The cellular 45Ca2+ content in the stimulated (60/min) ventricles of control rats (without Mn2+) was 0.83 +/- 0.09 mmol/kg wet weight (w.w.). Ten minutes of rest resulted in a gain of 0.06 mmol 45Ca/kg w.w. (not statistically significant). Fourteen days' exposure to Mn2+ resulted in an increase of the mean 45Ca content to 1.61 +/- 0.09 mmol/kg w.w. in the stimulated preparations and to 1.35 +/- 0.06 mmol/kg w.w. in the rested ones (p less than 0.001). Thus, the control rest preparations did not change their Ca2+ content, while in the rats treated with Mn2+ the rest resulted in an increase at exchangeable Ca by 52%. The maximal ventricular developed pressure (Pmax) after 14 days of Mn2+ administration was increased by 35% and dP/dtmax was 228% of the value in the control group.(ABSTRACT TRUNCATED AT 250 WORDS)