Metrifonate (0,0-dimethyl-[1-hydroxy-2,2,2-trichloroethyl]-phosphonate), and its rearrangment product dichlorvos (2,2-dichlorovinyl dimethyl phosphate) (DDVP), were studied in plasma from two patients with schistosomiasis who were treated with Bilarcil®. A mass fragmentographic technique was used. Isotopic variants of the substances were used as internal standards and to compensate for DDVP formed during the workup procedure. The results were related to erythrocyte and plasmacholinesterase determinations. The method described makes it possible to study pharmacokinetics in man and to relate this information to therapeutic effects. It is proposed that metrifonate acts as a slow release formulation for DDVP.