The plasma levels of somatostatin-like immunoreactivity, growth hormone and insulin were measured using a RIA method in healthy volunteers every 4h during the day and every 2h at night, without waking the subjects. In the waking state the fluctuation of plasma somatostatin-like immunoreactivity level only occurred near to meal time. A marked episodic surge of plasma SLI (peak value, 127.25 +/- 4.40 pg/ml (mean +/- ES)) was noted at 0200 in the initial period of slow wave sleep (SWS) 2h after the peak of GH. Insulin showed no sharp peak and its pattern was unrelated to other two hormones studied. A positive correlation was observed between SLI and GH in plasma using the mean cosinor method: the acrophase of SLI was at 0018 about 1h later than GH (at 2315). The acrophase of insulin occurred at 1525, significantly different as compared with the previous two. From these findings, it is concluded that SLI in peripheral plasma fluctuates with a significant circadian rhythm and a nychthemeral maxima as GH and that, whatever its source, that it is related to plasma GH and not to plasma insulin.