The growth of developing A/J mouse embryos was studied after maternal administration of phenytoin (Dilantin), an anticonvulsant drug. Wet weight, dry weight, protein and DNA contents of the embryos were quantitated 24 h after drug administration. Collagen content was investigated because of its importance in cellular differentiation. The wet and dry weight of embryos from phenytoin-treated mothers were 52.3 and 57.5%, respectively, of that of embryos of control mothers. DNA and protein contents were also decreased in embryos from phenytoin-treated mothers. Collagen represented only 0.07% of the protein present in day 11 control embryos, but was increased 4.9-fold in embryos from phenytoin-treated mothers in comparison to controls. These results suggest that phenytoin reduces overall embryonic growth but stimulates collagen synthesis.