The correlation between growth hormone (GH) secretion and the sleep-wakefulness cycle in the 29-31 day-old male rat was studied by serial blood sampling through an intracardiac cannula at 10 min intervals from 12.00 to 18.00 h. E.e.g.s recorded continuously during the blood sampling period were scored into wakefulness and sleep, and the amount of sleep for every 10 min was plotted against the GH value as assessed by radioimmunoassay. The power spectrum analysis and the least-squares method applied to the time series of GH concentrations in the control rat, revealed that the mean periodicity was 3.12 +/- 0.09 h (mean +/- S.E. of the mean) in fourteen rats examined. For the animals in which the e.e.g. was recorded simultaneously with blood samplings, the cross-correlation analysis performed between the time series of the amount of sleep and the GH value revealed a significant positive correlation between the GH level and the amount of sleep during the preceding 10 min. The time series of the sleep amount and the GH value had mean periodicities of 3.09 +/- 0.17 and 3.05 +/- 0.13 h (mean +/- S.E. of the mean), respectively, in twenty-seven rats examined. 5. Sleep deprivation performed between 13.00 and 15.00 h was effective in preventing GH secretory bursts which otherwise should appear during this time period of the day. 6. Corticosterone measurement by protein-binding assay revealed that the procedure of sleep deprivation did not act as the 'stress' which has been known to inhibit GH secretion in the rat. These findings indicate that GH secretion in the rat is sleep-related, as has been proved in the human.