The aim of the European Community Multicentre Trial on Fetal ECG Analysis During Labour was to collect a clinical database of electrocardiograms on which the different patterns of ST waveform and time interval changes, so far identified, could be studied. The aim of this paper was to study the PR-RR relationship and the P wave patterns during normal labour. One thousand three hundred fifty accelerations and 350 decelerations were analysed in 618 labours, all with normal fetal outcome. All ECGs were recorded with the STAN recorder and linked to a PC system for data acquisition and analysis. A positive relationship between P-R and R-R intervals was identified in all reactive fetal heart rate traces. A P-R shortening was identified also with all decelerations of more than 40 bpm from the baseline, resulting in a negative PR-RR relationship. Experimental works have described a P-R shortening concurrent with the maximal R-R lengthening during acutely induced hypoxemia in fetal lambs. In our study normal cord acid-base status indicated that the majority of bradycardia episodes recorded were not related to acute hypoxemia: PR-RR relationship changes seem therefore to indicate an intact physiological fetal heart adaptive response to rapid change in the environment of either hypoxemic or haemodynamic origin. A negative PR-RR relationship by itself seems to be nothing more than an indicator of decelerations during labour and seems unable to discriminate between decelerations of different origin. Finally the P wave was no longer detectable during decelerations of more than 800 msec: in this situation the P-R interval can not be reliably measured.