Rhesus monkeys were fed a diet marginally deficient in zinc (4 ppm zinc) throughout pregnancy and were monitored for changes in hematological, biochemical, and immunological parameters. This dietary zinc level was chosen because it did not produce an overt deficiency syndrome when fed for 10 wk to nonpregnant monkeys. Deprived animals were compared to control groups fed a zinc replete (100 ppm) diet ad libitum or on a food restricted (pair fed) basis. Beginning in the 3rd trimester zinc-deprived monkeys exhibited characteristic signs of deficiency including dermatitis, anorexia, and low levels of plasma zinc (less than 65 micrograms/100 ml) and of serum alkaline phosphatase activity. The extent of plasma zinc depression in deficient monkeys was dependent on total food intake; severely anorexic monkeys lost weight but maintained normal plasma zinc levels; monkeys that gained 20 to 30% of their body weight during pregnancy had severely depressed plasma zinc. Plasma vitamin A was reduced in the deprived group while copper, magnesium, and folate levels remained similar to controls. Hematological changes indicative of iron deficiency anemia (reduced packed cell volume, mean corpuscular volume, and Hb) were also seen in severely deficient monkeys. In addition, the peripheral lymphocyte mitogen response was reduced in deficient dams. We conclude that marginal deficiency of dietary zinc can produce significant abnormalities of nutritional status and has the potential for producing serious immunohematological dysfunction during pregnancy.