Predictions of human pesticide metabolism which are needed for the interpretation of biological monitoring data are frequently made from animal studies. Consequently, assumptions have to be made about the relationship between absorbed dose and metabolite excretion. The results from two human volunteer studies highlight the problems associated with extrapolating from animal studies in this way. The pyrethroid insecticide cypermethrin shows markedly different metabolite patterns when administered orally or dermally in man. Following dermal dosing the ratio of trans/cis cyclopropane acids is approximately 1:1, compared to 2:1 after oral administration. The ratio of total cyclopropane acids to phenoxybenzoic acids also differs depending on the route (dermal 1:4, oral 1:0.8). A knowledge of human metabolism by these two routes enables a much more meaningful interpretation of biological monitoring measurements. The herbicide molinate forms a mercapturate conjugate as a major urinary metabolite in the rat (35%). In volunteers at low dose levels this metabolite is present at insignificant levels (< 1%) and 4-hydroxymolinatc is a much more abundant metabolite (39%). This shows that extrapolation between species can be very misleading. It is concluded that the benefits of using human volunteers for metabolism studies at low doses far outweigh the minimal risks involved. As a basis for biological monitoring such studies can lead to a greatly improved risk assessment for pesticides in use.