Twenty-four hour secretory patterns of prolactin, luteinizing hormone (LH), and follicle-stimulating hormone (FSH) were obtained on two separate occasions from a woman with late physiologic lactation. The studies were performed 26 and 34 months after her child's birth. During the initial study, she had amenorrhea, and her child suckled 13 per cent of the 24 hour period (Study 1). At the time of the second study, she had resumed regular menses and her child suckled for 7 per cent of the 24 hour period (Study 2). The average concentrations of prolactin during Studies 1 and 2 were 40 +/- 1.0 (mean and standard error; No. = 72) and 31 +/- 1.4 ng. per milliliter, respectively. The mean plasma prolactin concentration in Study 1 was significantly greater than that in Study 2 (p less than 0.001). The plasma concentrations of LH and FSH were significantly less in Study 1 than in Study 2 (p less than 0.001 and less than 0.01, respectively). It is concluded that hyperprolactinemia and hypogonadotropinemia were endocrinologic correlates of the amenorrhea of late physiologic lactation in this woman.