We studied the effects of food intake and lactation on the duration of postpartum infecundability in rats. Sprague-Dawley rats were given free access to a purified diet (AIN-76A) or a similar diet (modified to contain twice the concentrations of vitamins and minerals) in amounts equal to 50% of that consumed by controls. At -65 d of age, animals were bred. At parturition, dams were allowed to nurse 4-5 pups, or all pups were removed. Thus, four groups were created: lactating control (n = 11), nonlactating control (n = 11), lactating food restricted (n = 8), and nonlactating food restricted (n = 10). Vaginal cytology was observed twice daily for the detection of proestrus, which occurred at 4.5 +/- 0.5 and 6.7 +/- 0.5 d (mean +/- SD) postpartum in nonlactating control and food restricted groups, respectively (P less than 0.005), and at 16.5 +/- 0.5 and 28.8 +/- 0.5 d postpartum in lactating control and food restricted groups, respectively (P less than 0.0001). The interaction between food intake and lactation was significant (P less than 0.0001). These results indicate that maternal food restriction exacerbates the effects of lactation in prolonging postpartum anestrus.