1. The secretion of prolactin and growth-hormone (hGH) was investigated during sleep in 10 healthy volunteers (8 males and 2 females): The comparison of one baseline night, one night after daytime physical exercise, and one night with selective deprivation of sleep stages 3 and 4 and paradoxical sleep showed clear differences of prolactin and hGH secretion during sleep. 2. Prolactin secretion is entrained into the sleep cycle of Non-REM and REM periods. A maximum of plasma hormone elevations occurs during the first quarter of sleep cycles, i.e., during Non-REM periods and less frequent rises at the end of the cycles, mainly during REM periods. 3. In contrast to growth hormone, concentrations of prolactin remain high also during later cycles occurring toward morning. This shows that high prolactin, but not high concentrations of hGH, regularly occur during sleep cycles with small amounts of slow-wave sleep. 4. Maximal prolactin concentrations during sleep are affected neither by preceding daytime physical exercise nor by selective deprivation of slow sleep stages 3 and 4. This is further evidence that slow-wave sleep stages are not necessary for the development of high plasma prolactin concentrations. However, peak values of growth hormone in the first and second cycle are significantly diminished after selective deprivation of sleep stages 3 and 4. 5. In abnormally long sleep cycles with artificial delay of the first REM period, the cyclical rhythmicity of prolactin release seems disturbed. This is further evidence for the sleep-dependent rhythmicity in the secretion of this hormone.