Normal pubertal development in humans involves two distinct processes: maturation of adrenal androgen secretion (adrenarche) and activation of the hypothalamic-pituitary-gonadal axis (gonadarche). One factor thought to contribute to the adrenarche in man is increased adrenal 17-hydroxylase (CYP17) activity. In the rat, there is evidence for adrenal involvement in the initiation of puberty, but the adrenal glands of this species are generally thought to express CYP17 only very poorly at best. To further examine the nature of postnatal adrenal development in rat, plasma samples and adrenal tissues were taken from animals aged 2-90 days, circulating adrenal steroids assayed, and adrenal zones assessed quantitatively. A relative increase in zona reticularis, and peaks of circulating cortisol, androstenedione, and 17-OH-progesterone were observed around postnatal days 16-20, clearly before the development of the gonads, which begins at 30-35 days. Quantitative reverse transcriptase PCR confirmed a peak in mRNA coding for CYP17 in adrenal tissue from rats of similar age. The results suggest that the rat adrenal has the capacity to secrete steroids arising from 17-hydroxylation, and that this may contribute to a process similar to human adrenarche.