1. Metabolites (RS)-4-[(3-hydroxy-2-methoxy)propoxycarbonylamino]butanoic acid (I) and (RS)-2-[(3-hydroxy-2-methoxy)propoxycarbonylamino]acetic acid(II) were isolated from urine after i.v. administration of (RS)-2-methoxy-3-(octadecyl-[14C]carbamoyloxy)propyl 2-(3-thiazolio)ethyl phosphate (14C-MOTP) to rats and characterized by t.l.c., g.l.c.-mass spectrometry and p.m.r. spectrometry. 2. After i.v. administration of 14C-MOTP, the plasma concentration of the drug declined biphasically with half-lives of 0.22 and 3.94 h in rats, and 0.81 and 8.00 h in dogs. In rats and dogs, unchanged MOTP was the main 14C component in the plasma, together with a small amount of I and II. 14C-MOTP was highly bound to plasma protein of both animals. 3. Five min after i.v. administration of 14C-MOTP to rats, 14C was widely distributed in tissues, with the highest conc. in the lung and the lowest in the eye. The distribution of 14C was relatively slow in some tissues. In most tissues, 14C decreased to low levels at 96 h, except in the Harder's gland. 4. Elimination of 14C-MOTP was almost complete within 120 h in rats and 144 h in dogs. In both species, the administered 14C was excreted largely in the urine as I and II, with the remainder appearing in the faeces and the expired air. Biliary excretion and reabsorption of 14C were detected in rats. 5. During repeated i.v. administration of 14C-MOTP to rats for 7 days, the conc. of 14C in plasma and most tissues attained steady state within 5 days, except in Harder's gland, where the level rose gradually until the seventh day of dosing. Within 6 days after the last dosing, 96% of the injected dose was eliminated from the body.