The effects of intramuscularly administered pethidine on indices of fetal heart rate (FHR) variability were studied during labour in 20 women. The differential index, describing the short term variability, and the interval index, describing the long-term variability, were measured from direct fetal electrocardiograms by a micro-processor-based 'on-line' system. The indices were measured form 10-minute samples, one before the pethidine injection and nine times after the injection. The interval index decreased significantly (p less than 0.01) with the maximum effect 40 minutes after the injection. The differential index also showed a decreasing trend, with a maximum at 40 minutes, but the change was not significant. Both indices returned to the pre-injection level after 60 minutes. The effect of pethidine on FHR variability during labour might cause difficulties in assessing the fetal state.