Metabolism of carbaryl by rat liver and skin post-mitochondrial fraction has been measured in the presence and absence of cofactors to promote different metabolic pathways. The metabolic capacity was compared with the metabolism of carbaryl during percutaneous absorption in a static skin diffusion system using a variety of receptor fluids. Carbaryl was metabolised by hydrolysis, and ring hydroxylation followed by conjugation to the glucuronide or sulphate with liver post-mitochondrial fraction. Using skin post-mitochondrial fraction only hydrolysis and conjugation were detected. No metabolism was seen during percutaneous absorption in vitro even with receptor fluids which maintain the skin tissue viability. Studies using post-mitochondrial fraction indicate the metabolic capacity of the tissue, whereas during absorption, rates of absorption and accessibility of substrate to the metabolising enzymes must be considered.