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Department of Physiology, Monash University, Clayton, Victoria, Australia.

1. We have studied the effect of altering the time of darkness during a 12 h photoperiod on the diurnal rhythms of maternal and fetal plasma melatonin and prolactin concentrations and fetal breathing movements (FBMs) in the late pregnant sheep. 2. Four ewes were exposed to a 'normal' 12 h light-12 h dark regime with lights off at 19.00 h. Eight ewes were exposed to an 'altered' 12 h light-12 h dark regime with lights off at 11.00 h. FBMs were recorded continuously and 2 hourly maternal and fetal blood samples taken during 24 h experiments between 120 and 138 days gestation. 3. Plasma melatonin concentrations were higher during the hours of darkness in the ewes and fetuses in both the normal and altered groups; i.e. altering the time of darkness was associated with a shift in the timing of the daily increase in melatonin concentrations in the mother and fetus. 4. In the fetuses, the mean 24 h plasma prolactin concentration was higher in the altered lighting group (28.8 +/- 6.1 micrograms/l) compared to the normal lighting group (13.0 +/- 3.9 micrograms/l). There was no difference, however, between groups in the daily variation of fetal plasma prolactin concentrations, which were higher at 17.00-01.00 h than at 05.00-19.00 h. Thus, altering the time of darkness did not change the timing of the daily plasma prolactin rhythm in the fetus. 5. Under the normal lighting regime maternal plasma prolactin concentrations were higher at 17.00-19.00 h (i.e. from 2 h before lights off) than at 09.00-11.00 h. In contrast, under altered lighting, plasma prolactin concentrations were higher at 01.00-07.00 h (i.e. 2-8 h after lights on) than at 09.00 h. 6. In the normal lighting group the incidence (min/h) of FBMs were highest at 16.00-21.00 h and reached a minimum at 05.00-06.00 h. In contrast, in the altered lighting group the incidence of FBMs was lowest at 19.00-20.00 h and reached a maximum at 11.00-12.00 h. 7. The results suggest that the light-dark cycle influences the diurnal modulation of FBMs, but not the daily variation of fetal plasma prolactin concentrations. The role of plasma melatonin concentrations, and evidence for a photo-inducible phase of increased prolactin secretion in the fetus, is discussed.