The cycle ergometer (an exercise cycle) Monark was used to study the repercussions on the fetus of physical exercises carried out by the mother at the end of pregnancy (after 37-39 weeks of amenorrhoea). The maternal and fetal heart rates were measured at the same time while the mother was pedalling against increasing resistance. This was a form of sub-maximal aerobics. Fourteen women took part in the study but only six traces could be analysed. The mean duration of the exercises was 15 (+/- 2.5) minutes. The mothers' heart rate rose from 84 (+/- 7) beats a minute (bpm) before exercise to 153 (+/- 14) bpm at the last level reached. It took 15.6 (+/- 4) minutes for the maternal heart rate to return to what it was before exercise. The basal fetal heart rate was around 144 (+/- 5) bpm before exercise and rose to 170 (+/- 12) during exercise with no change in the oscillations in the fetal heart rate. After exercise the fetal rate slowed down very slowly and there was a marked diminution in the amplitude of the oscillations (Hammacher's Type 1). They never disappeared. Those few women who were in "false labour" during the test went out of false labour. We have hypothesised that the fetal heart rate rises significantly during the mother's exercise and this action is a mechanical one rather than a humoral or hormonal one. When exercise stops the diminution in oscillations, which is rapid, could be a sign of the fetus becoming drowsy, or it could be due to the intervention of maternal factors.