The expression of argyrophilic nucleolar organizer regions (AgNORs) was studied in the anterior lobe of the hypophysis of 1-, 7-, 14-, 28- and 80-day-old rats. Mean area and numbers of AgNORs per nucleus decreased during this period with a simultaneous increase of both nuclear area and mean AgNOR-particle area. Comparison of our data on AgNOR expression with previously reported data on the proliferative activity and cell maturation and differentiation of anterior pituitary cells during a similar period led us to conclude that, at least in our experimental model, variations in the expression of AgNORs are more related to phenomena of cellular maturation than to proliferative activity.