To study the secretory pattern of growth hormone (GH) in the adult male rat in a continuous sleep state, the GH concentration was measured in blood sampled at 10-min intervals through an intracardiac cannula for 9–10 h long-term anesthesia with thiopental sodium. The cortical EEG was monitored to maintain stable EEG sleep throughout the sampling period. In 41 animals subjectd to anesthesia, 32 showed an apparent fluctuation in GH levels. The analysis of the time series of the GH concentration in blood by the power spectrum and least-squares method revealed that 27 animals had periodicities within the mean ± 2SD range obtained for the unanesthetized, but similarly sampled, animals from 2.40 to 3.85 h. The mean (± SE) of the periodicities for 27 animals was 3.01 ± 0.06 h, similar to that obtained for the unanesthetized animals, approximately 3.0 h. The results indicate that, although the GH-controlling mechanism in the rat is facilitated under sleep, it has a 3.0-hour period ultradian rhythm mechanism that is indepedent from that for s