Female rats were submitted to light-darkness, to constant light on day 60 or to constant light from birth. Plasma prolactin was measured on days 80,100 and 120 in all experimental situations before ovariectomy was performed. 10 days later, a blood sample was taken and the animals were injected subcutaneously with 75 µg of estradiol benzoate. Blood was again drawn on 2 consecutive days. Results showed that: (a) prolactin levels were higher in the groups submitted to prolonged periods of constant light (60–120 days); (b) after ovariectomy prolactin levels decrease but remain higher in these same groups, and (c) constant light produced a higher prolactin response to estradiol benzoate. These data indicate that the length of exposure to constant light may be a critical factor in the development of alterations in the control of prolactin secretion.