The influence of the organochlorine insecticides dieldrin and DDT on the normal development of carbohydrate and lipid metaholism in the neonatal rat was studied using measurements of the in vitro activity of hepatic enzymes, and plasma and hepatic metabolites, during, and subsequent to, the suckling period, after maternal feeding of DDT (20ppm) or dieldrin (2ppm) . The influence of short term insecticide exposure on the rate of glucose turnover in vivo, and gluconeogenesis in the perfused liver were also investigated. In addition, the pharmacokinetics of these compounds in maternal and neonatal tissues were investigated. The effects of dieldrin on the normal developmental changes occurring in the neonate were not large, and did not persist beyond the early post-weaning period. DDT induced larger modifications in the developmental pattern of some enzymes, which were still evident at 10 weeks of age. However, they had largely disappeared by 20 weeks. An hypothesis based on a disturbance by DDT of the insulin / glucagon / cyclic AMP system was developed, but was not supported by further chronic or acute exposure studies. It was shown that both DDT and dieldrin accumulated rapidly in maternal tissues. Although the foetus received some insecticide via the placenta, this tissue appeared to serve a protective function, and the highest exposure occurred in the post-natal period, due to the high rate of secretion of these compounds into milk. The use of developmental enzyme pathology as a predictive tool in toxicology was discussed, with particular reference to the need for early markers for toxic effects. It was concluded that the organochlorine insecticides, although very persistent, may nevertheless continue to be of great value if used responsibly, and may be safer than the more recently favoured organophosphorus and carbamate insecticides.