The toxicity of organic solvents at different concentrations was examined to determine their effect on pregnant rats and their offspring. A very serious effect which inhalational exposure to n-hexane, methyl-ethyl-ketone and a mixture of the two organic solvents produced in pregnant rats was a concentration-dependent increase of intrauterine mortality. In cases of only prenatal exposure, the persistence of reduced body growth was an important result obtained at all concentrations we investigated (first solvent experiment, 500 ppm; second solvent experiment, 800 ppm; and third solvent experiment, 1000 to 1500 ppm; 23 hr day). In these cases, a delay in the maturation of cerebellar cortex was also observed. In cases of pre- and postnatal solvent exposure, the effects of malnutrition were added to the solvent-induced retardation. This resulted in an extreme delay in tissue maturation accompanied by a retarded cell maturation. The solvents examined had no teratogenic effect in pregnant rodents and their offspring. Neurotoxicity was restricted to the known structures of the adult animals, whose axons were damaged. They did, however, show a concentration-dependent embryotoxic and fetotoxic effect.