The implantation failure in newly inseminated mice induced by food deprivation for 48 hr, beginning at 0900 hrs on day 4 post coitum, was prevented by simultaneous exposure to light continuously for 48 or 36 hr. Food-deprived females that were exposed to continuous light for 36 hr showed a significant increase in fetal resorption as compared with food-deprived females exposed to continuous light for 48 hr. Since failure of hypophysial prolactin release appears to be the primary endocrine cause of the inanition-induced implantation failure, the results suggest that exposure to continuous light protects implantation in food-deprived females by stimulating luteotrophic activity.