Localization of calretinin immunoreactivity in the rat adrenal gland was studied using immunohistochemical methods. Calretinin-immunoreactive adrenal chromaffin cells and nerve fibres were found at all of the ages examined from embryonic day 16 up to 2-yr-old. Immunoreactive chromaffin cells showed a decrease in number with increasing age, and in the adult and ageing rats they were found among the adrenaline-storing chromaffin cells. Calretinin immunofluorescence was also observed in a few ganglion cells of the adult and ageing rats, and in most of the cortical cells at embryonic day 16. The density of immunoreactive nerve fibres in the gland showed a progressive increase with age to a peak by day 4 after birth and then a gradual decrease afterwards; they became associated mainly with some of the ganglion cells in the adult rat. As revealed by double labelling of calretinin immunoreactivity in adrenal sections from the adult rat with NADPH-diaphorase histochemistry, no co-localization existed between calretinin and NADPH-diaphorase; however, a small number of neuronal cell bodies which contained nitric oxide synthase were found heavily surrounded with calretinin-immunoreactive nerve fibres. In ageing rats, an increased density of calretinin-immunoreactive nerve fibres was seen associated largely with the chromaffin cells. We conclude from this study that calretinin is found in the rat adrenal gland; the high degree of variability in its expression with age suggests that calretinin may be concerned with age-related activities in the gland.