The relationship between aging changes in pituitary-testicular function and the penile erection reflex was examined in middle-aged male rats. Significant decline in circulating LH and testosterone is first apparent in male rats during middle-age (13-15 mos). In Experiment 1, middle-aged (13-15 mos) and young (3-4 mos) rats were tested for erections responses. The erections were measured in animals held supine and mechanically stimulated by retraction of the penile shealth. Middle-aged rats had substantially fewer erections than young animals. In Experiment 2, groups of middle-aged and young animals were castrated and provided equivalent levels of circulating testosterone (T) through the use of Silastic capsules. The levels were measured directly by radioimmunoassay of plasma T. In animals with supraphysiological T levels, erection frequency was low, but there was no significant difference between middle-aged and young animals. The results suggest that middle-aged rats do not have irreversible deficiencies in the somatic and neural mechanisms regulating erection nor are they significantly less responsive to androgen stimulation. The decline in circulating testosterone with age may therefore be a primary factor in the reduction of penile erections in middle-aged rats.