The investigations were carried out in 50 young healthy men, of whom 20 made an increasing submaximal physical effort on a bicycle ergometer after drinking a 250 ml coffee infusion (20 g of pure natural coffee); the remaining 30 men represented the control group. The left ventricular systolic time intervals (LVSTI) were obtained from polycardiograms by means of the Weissler et al. method. It was found that 20 minutes after coffee drinking the left ventricular ejection time index (LVETI) and LVET/PEP ratio increased whereas the pre-ejection period index (PEPI) and the isovolumic contraction time (ICT) decreased. This improvement of the activity of the heart as a pump which is indirect proof of increased myocardial contractility after drinking coffee was especially noticeable during subsequent phases of physical effort. The recovery period in group 1 was characterized by a reduced PEPI and ICT and increased LVETI and LVETI/PEP ratio as compared with the control group.