Rapid eye movement (REM) sleep deprivation leads to an induction of galanin gene expression in the rat brain, especially in the hypothalamus. Galanin affects neuroendocrine systems that are involved in sleep regulation, i.e. the growth hormone-releasing hormone-dependent system of the hypothalamus and the locus coeruleus. In the study reported here we investigated the effects of 4 x 50 microg galanin (n = 10) and of 4 x 150 microg galanin (n = 8) administered hourly between 22.00 and 01.00 h as intravenous boluses on the sleep EEG and nocturnal hormone secretion in healthy young men. Galanin administration significantly increased REM sleep in the third sleep cycle with no difference between the two doses. Spectral analysis revealed a significant increase in the EEG power in the delta and theta frequency range for the total night after the lower dose of galanin, but not after the higher dose. The secretion of growth hormone, cortisol and prolactin remained unchanged during sleep in both cases. Our data are consistent with the assumption of a functional resemblance between the effect of galanin and that of REM sleep deprivation, which is known to have antidepressive efficacy.