The levels of specific soluble aspartyl aminopeptidase activity were assayed in retina, occipital cortex, anterior pituitary, posterior pituitary, pineal gland and serum of adult male rats, using Asp-2-naphthylamide as substrate, in a 12:12 h light:dark cycle (7-19 h light). Significant diurnal variations appeared in retina, pineal gland, occipital cortex and serum. In addition, different patterns of diurnal variation of the enzymatic activity were observed in the tissues analyzed. A regular increase of the activity was noticed at the end of the dark period in all the tissues as a common feature, except in serum, in which the enzymatic activity reached a peak in the middle of the light period, decreasing progressively during the dark hours. After the last hours of darkness, the pattern of variation in the activity differed in each tissue. These diurnal variations in aspartyl aminopeptidase activity could reflect the functional status of its putative endogenous substrates, such as angiotensin II, and it may also suggest the existence of differential regulatory mechanisms associated with each location.