The effects on parturition of a low zinc diet during pregnancy and of three levels of zinc repletion late in pregnancy were compared in two strains of rats, Sprague Dawley and Long Evans. Long Evans rats had a higher hematocrit and higher plasma protein levels than did Sprague Dawley rats. Long Evans rats also had heavier mammary glands and tended to deliver somewhat later than the Sprague Dawley rats. Weight gains during pregnancy and weights of the liver, spleen, adrenal glands, and pups did not vary between strains. No differences were detectable in the degree of stress at parturition between the unsupplemented females of the two strains. Injection of 300, 600, or 900 microgram zinc on day 18 of pregnancy increased female weight gains, pup weight, and spleen and mammary gland weights, and decreased hematocrit level. Three females in the unsupplemented groups died about the expected time of parturition; none died in the supplemented groups. Although there were some differences between the strains, the effects of a low zinc diet and the requirement for zinc for normal parturition were the same in the two strains. An injection of 900 microgram zinc on day 18 resulted in normal parturition in both strains.