Concentrations of LH, FSH and prolactin (PRL) were measured by radioimmunoassay throughout pregnancy (day of vaginal plug = day 0) in mice exposed to light for 14 hr (on between 0500–1900 hr). During the preimplantation period an increase in LH occurs, with a peak on day 3. Another peak appears during midpregnancy, with the greatest level of LH appearing on day 10 (37 ± 12 ng/ml). A comparable level was found only on the day of parturition. Levels of FSH were greatest on day 0 and on the day of parturition. FSH concentrations were low (Endocrinology94: 112, 1974)