To elucidate the mechanism of the resting-active cycles (RAC) of fetal heart rates (FHR), in the resting and non-resting phases (RP and NRP), 24-h FHR recordings were made on 16 normal full-term pregnant women. RP, NRP, RAC-1 (NRP-NRP cycle), and RAC-2 (RP-RP cycle) were defined based on the criteria of Nijhuis et al. Frequency distributions were plotted separately for the entire 24-h period as well as for the day-time (07:00-21:00 h) and night-time (21:00-07:00 h), and were compared using Kolmogorov-Smirnov two-sample tests. The mean durations (+/- S.D.) (min) of RP, NRP, RAC-1, and RAC-2 were 22.7 +/- 11.2, 67.3 +/- 47.2, 90.0 +/- 47.6, and 89.9 +/- 48.6 during 24-h periods, 20.1 +/- 7.7, 68.3 +/- 52.3, 88.6 +/- 53.1, and 88.4 +/- 53.0 during the day-time, and 25.4 +/- 13.2, 66.2 +/- 41.2, 91.4 +/- 41.0, and 91.5 +/- 43.4 during the night-time. Length of RP was the only factor significantly different during the day and night (P < 0.05). We propose that there are different mechanisms controlling RP and NRP.