Previous research from this laboratory has shown that triethylenetetramine (TETA) fed to rats throughout pregnancy at 0.83% or 1.67% of the diet is teratogenic and results in low copper and high zinc levels in material and fetal tissues. These results suggested that the teratogenic effects of TETA were due to copper deficiency and/or zinc toxicity induced by the drug. In the present study, dams were fed TETA in a control (5 micrograms copper/gm) or copper supplemented (50 micrograms copper/gm) diet throughout pregnancy. Fetuses were removed on day 21 of gestation and examined for abnormalities. Copper supplementation reduced the teratogenicity of TETA; the reduction was correlated with an increase in maternal and fetal tissue copper levels. Copper supplementation did not alter the effects of TETA on tissue zinc levels. Thus TETA teratogenicity appears to be due primarily to induced copper deficiency, which may be reduced by dietary copper supplementation.