Overnight serum insulin and plasma glucose concentrations were measured every 20 min from midnight to 0900 h in 13 nondiabetic subjects. Seven were normal men, and 6 had isolated GH deficiency. The pre-breakfast increase in serum insulin concentrations ("dawn phenomenon") did not occur in either group of individuals, and a progressive decline in serum insulin concentrations occurred particularly in the dawn hours (0600-0900 h). The GH secretory patterns were strikingly different in the two groups, with normal spontaneous GH peaks mostly between 0100-0200 h in the normal subjects vs. virtually flat GH secretion in the isolated GH deficiency group. The absence of the dawn phenomenon in these nondiabetic subjects regardless of their GH secretory pattern suggests that the dawn phenomenon, as described in insulin-dependent diabetic patients, is not an exaggeration of normal circadian rhythmicity in insulin sensitivity.