The activity of five acid hydrolases in the adrenal gland at the perinatal stage in adult rats was measured here and changes in α-mannosidase and N-acetyl-β-D-glucosaminidase activity were detected. These enzymes increase after birth reaching a peak between days 4 and 7. Other enzymes such as β-glucuronidase, arylsulfatase and β-glucosidase did not significantly change at the ages studied. These data suggest that the enzymatic activity and development of the adrenal gland may be correlated during the first week after birth; this is critical since most of the changes occur in this organ.