Tryptophan administration was used to evaluate the possibility that serotonergic neurons are involved in regulating the release of cortisol, renin, and aldosterone. Eleven studies were undertaken using 2 or 10 g tryptophan administered to fasting patients in continued supine posture. Aldosterone rose significantly to 208% (range, 128-329%) of baseline in all seven studies using 10 g and in one of the four studies using 2 g. Renin rose significantly to 189% (range, 116-340%) of baseline in four of the seven studies using 10 g and in two of the four studies using 2 g. Cortisol rose from 10.1 +/- 3.3 to 20.0 +/- 3.7 micrograms/100 ml (P less than 0.001 by t test) in six of the seven studies using 10 g and three of the four studies using 2 g. In eight studies, there was a significant rise of more than one substance after tryptophan administration. In six of these, peak values of the responding hormones occurred at the same time or within a single 30-min sampling interval despite the absence of a constant relationship between their rises. The results suggest participation of the central serotonergic nervous system in the control of renin and aldosterone in addition to its previously postulated role in cortisol release.