Corticosterone concentrations and corticosterone binding in the serum were studied in immature female rats, using radioimmunoassay and batchwise gel equilibrium techniques. A parallel developmental pattern was found for corticosterone levels and its serum binding with a neonatal drop, followed by low levels until 12 days of age and a rise between 12 and 28 days of age. Effects of adrenalectomy, of ovariectomy, of the combined operation and of sham-operations, performed at various ages, were also studied. Adrenalectomy performed at 5 days of age did not decrease serum corticosterone concentrations within a 6-day period whereas it did in older rats. Complete disappearance of corticosterone from the blood occurred only in adult rats after combined adrenalectomy/ovariectomy. Ovariectomy and sham-operations in the younger age groups (5-15 days) caused a gradual increase in corticosterone concentration with maximal values 6 days after operation or later. The response of corticosterone secretion to these operations became more moderate and quicker, i.e. more adult-like, at 28 days of age, the age where corticosterone concentrations in intact rats also seemed to reach a plateau at an adult-like level. Corticosterone binding changed only marginally after ovariectomy or sham-operations until 28 days of age, when an increase was induced by these operations. After adrenalectomy or combined adrenalectomy/ovariectomy, however, marked increases in serum binding of corticosterone were always seen. In summary: though a parallel developmental pattern of serum corticosterone levels and corticosterone binding was seen in the maturing rat, interference with the normal condition causes divergent responses in these two parameters. Moreover, the responses vary with maturational age.