Data on reproductive functions in a group of 20 undernourished beef heifers were compared with concomitant data obtained from 20 control heifers. Both groups were fed the same diet, but the amount of the diet received by the undernourished heifers provided only about 85% of the maintenance requirements for energy and protein. Undernutrition began on day 5 of an estrous cycle and continued until the animals were slaughtered 3 to 18 days after mating. Undernutrition reduced plasma levels of progesterone within 5 days, temporarily reduced the number of medium-sized ovarian follicles, altered the length of the estrous cycle, and reduced the proportion of heifers with normal fertilized ova. Weights of corpora lutea formed in undernourished heifers were about 70% of control values.