To determine the role of number and position of chlorine substituents on the developmental toxicity of diphenyl ether analogs of nitrofen, we have evaluated one unchlorinated, three monochlorinated, and five dichlorinated-phenyl 4'-nitrophenyl ethers with respect to effects on liver weight and on maintenance of pregnancy in females, and with respect to postnatal survival and the occurrence of small or absent Harderian glands in offspring. None of the diphenyl ethers evaluated in these experiments was as active as nitrofen with respect to any parameter evaluated. Both the position and the number of chlorine substituents affected toxicity, but no simple relationship between number or position of chlorine substituents and either maternal or fetal endpoints was established.