Plasma concentrations of prolactin and corticosterone were determined in hourly samples collected over a 25-hr period from unrestrained turkey hens exhibiting incubation (broody) behavior. Hens were maintained in cages on a 14 hr light: 10 hr dark photoperiod. Mean plasma prolactin concentration increased significantly late in the dark period to reach maximum daily levels of 496 ng/ml at the start of the photophase. However, a well-defined daily rhythm in prolactin secretion was not evident. Plasma corticosterone concentration showed a significant daily rhythm characterized by a major secretory peak (7.14 ng/ml) in the middle of the light period and a smaller, less well-defined peak (4.11 ng/ml) during the dark period. Both prolactin and corticosterone secretion ranged widely throughout the day in individual hens in a manner suggestive of pulsatile hormone secretion. This study demonstrates that the extremely high prolactin concentrations characteristic of the incubating turkey hen reflect large, dynamic changes in plasma prolactin concentrations occurring throughout the photoperiod, rather than a constantly high level of prolactin secretion.