The secretory pattern of GH in the mature rat is sexually differentiated. In male rats GH is secreted in pulses occurring at regular 3- to 4-h intervals. In females the pulses are lower and plasma GH levels between the pulses are higher than in males. The continuous presence of testosterone appears to be necessary to maintain low basal GH levels in adult male rats. Neonatal, but not prepubertal, gonadectomy decreases GH pulse height in adult male rats to female levels. Administration of testosterone neonatally to castrated animals returns GH pulse height to normal suggesting that neonatal testicular androgen secretion is one determinant for GH pulse height in adult male rats. Administration of testosterone neonatally or during adult life to neonatally ovariectomized rats also produces higher GH pulses. In contrast to testosterone, estrogens elevate basal plasma GH levels and suppress the GH pulses under some conditions. Estrogens may stimulate basal GH secretion by acting directly on the pituitary. The physiological significance of the secretory pattern of GH has been investigated in hypophysectomized rats by simulating different plasma patterns of GH. The results suggest that high, infrequent GH pulses with low plasma GH levels in between (i.e. a masculine plasma GH pattern) promotes growth more effectively than an intermediate, rather constant level of plasma GH (i.e. a feminine plasma GH pattern). Since male sex steroids masculinize the secretory pattern of GH and have only minor growth-promoting effects in hypophysectomized animals it appears that the growth promoting effect of androgens is indirect and is due to an altered secretory pattern of GH. Presumably, neonatal androgen secretion stimulates body growth during adult life by irreversibly masculinizing the secretory pattern of GH. In contrast, estrogens appear to influence body growth by mechanisms that are mainly independent of the secretory pattern of GH. Evidence is accumulating that the secretory pattern of GH in the rat also affects various sexually differentiated hepatic characteristics such as steroid metabolism and prolactin receptor concentration. Thus, a feminization of the liver develops after continuous, but not intermittent, administration of GH to hypophysectomized rats. GH secretion is predominantly regulated by two hypothalamic peptides; GRF, and the GH-release-inhibiting factor, somatostatin.(ABSTRACT TRUNCATED AT 400 WORDS)