Several commercial solvent mixtures commonly used as insecticide carriers in spray formulations increase by more than threefold the microsomal N -demethylation of p -chloro N -methylaniline in midgut preparations of southern armyworm (Spodoptera eridania) larvae exposed orally to the test solvents. Under laboratory conditions, the same solvent mixtures exhibit a protective action against the in vivo toxicity of the insecticide carbaryl to the larvae. The data are discussed with respect to possible solvent-insecticide interactions occurring under field conditions and, more broadly, to potential toxicological hazards of these solvents to humans.