To analyze the secretion patterns of LH, FSH, GH, and PRL in the late gestational sheep fetus in vivo, we measured simultaneous plasma levels of these hormones during a period of frequent sampling under basal conditions (samples every 15 min for 5 h) in 17 chronically catheterized sheep fetuses. To calculate mean plasma levels and areas under the curve, we analyzed hormone pulses and coincident pulse patterns to assess interactions between the release of these pituitary hormones. Mean plasma levels for all fetuses were: LH, 0.8 +/- 0.2 ng/ml (mean +/- SEM); FSH, 4.6 +/- 0.7 ng/ml; GH, 136.6 +/- 16.5 ng/ml; and PRL, 40.5 +/- 10.3 ng/ml. Pulse analysis detected 20 LH pulses during 5100 min of total sampling time, which gave a mean interpulse interval of 255.0 min. For GH, 37 pulses were detected; the mean interpulse interval was 129.7 min. Twenty PRL pulses yielded a mean interpulse interval of 225.0 min. FSH pulses could not be analyzed due to the long half-life of this hormone, but hormone level fluctuations were screened for maxima. A new method was developed to detect an interaction between hormone pulses. The probability of the simultaneous occurrence of hormone pulses was calculated and compared with the rate of coincidences found in the experiments. Analysis of copulsatile release of LH, GH, and PRL revealed 11 GH pulses coinciding with the LH pulses (P = 0.0020). An interaction between the pulsatile release of LH and GH can, therefore, be assumed. There was also a significant interaction between GH and PRL. Seven PRL pulses preceded the GH pulses by 15 min (P = 0.0014). In contrast, no significant copulsatile release could be observed between LH and PRL; 95.5% of LH pulses were accompanied by a maximum FSH level, suggesting an interaction between LH and FSH secretion. In summary, we show that LH, GH, and PRL (and possibly FSH) are secreted in a pulsatile fashion in the ovine fetus. Furthermore, the pulsatile releases of LH, FSH, and GH as well as GH and PRL are temporarily coupled, as demonstrated by a significant number of coincident pulses between LH/GH and GH/PRL and a high number of FSH hormone maxima concomitant with LH pulses.