We have studied the development of the adrenal gland in the rat comprising the ages ranging from 0 to 90 days after birth. The weight of the animals and that of the adrenal glands demonstrated a linear growth with time until 75 days, both in males and females. The area of the zona glomerulosa (ZG) increased in size from birth until approximately 40 days of age. After that, growth had a much smaller slope (females, r=0.84, P < 0.001; males, r=0.81, P < 0.001). Aldosterone secretion had a marked increase until 20 days of age and thereafter demonstrated a tendency for a decrease (females, r=-0.19, P < 0.02: males r=-0.26, P < 0.001). Plasma renin activity followed a trend parallel to that of aldosterone. The steroid precursor 18-OH-deoxycorticosterone (18-OH-DOC) demonstrated a different course as it increased progressively with age especially in the females (females, r=0.57, P < 0.001; males, r=0.40, P <0.001). The expression of the enzyme 3-beta-hydroxysteroid dehydrogenase (3-beta-HSD) was also studied by immunohistochemistry and it was shown to be very low at birth and starting to increase by 10 days of age. After 30/40 days of age the amount of this enzyme existing in the ZG was comparable with that of the outer zona fasciculata (ZF). We conclude that the development of the ZG in the rat has particularities that make it different from that of the rest of the cortex.