Summary. Pregnant mice were treated with antiserum to LH or bromocriptine to inhibit the activity of LH and prolactin, respectively. Luteal function was monitored by the radioimmunoassay of plasma progesterone. Bromocriptine treatment on Days 2 or 5 of pregnancy produced a rapid decrease in progesterone secretion, but had no effect on luteal function when given on Days 6, 7 or 8 of gestation. Treatment with LH antiserum before implantation did not inhibit progesterone secretion, but luteal function was severely impaired when the antiserum was given on Days 5–9 of pregnancy. These results demonstrate the dynamic nature of luteal dependency on prolactin and LH, and indicate that LH is an essential component of the luteotrophic complex of the mouse.