Responsiveness to caffeine was studied in early human fetal hearts surviving in nutrient solution. Alterations of the spontaneous contraction rate were followed. Three concentrations (0.01, 0.1 and 1 mmol/l) were applied. In the presence of caffeine there was a significant age- and concentration-dependent increase in fetal heart rate in the period of the 6th to 17th weeks of gestation. Concerning the caffeine consumption habits of pregnant women a questionaire investigation was carried out in 686 cases. 69 per cent of the pregnants regularly drink coffee; 10.7 per cent of them consumes more than 300 mg per day. The authors call attention to the potential harmful effect of caffeine on the fetal heart.