The new choleretic ethyl (Z)-(3-ethyl-4-oxo-5-piperidino-thiazolidin-2-ylidene)-acetate (piprozoline, Gö 919, Probilin), a monosubstance of the substituted methylthiazolidones, was investigated by thin-layer chromatography with regard to pharmacokinetics in the rat (100 mg/kg i.v.; 20 mg/kg i.v.), dog (100 mg/kg i.g.; 20 mg/kg i.v.) and man (200 mg orally in sugar coated tablets). The substance was absorbed quickly in all species. Because of its rapid metabolisation, unchanged substance could be detected only in the dog after oral administration. In this case and after intravenous administration of piprozoline, the plasma level of the unchanged substance cold be described by two-compartment models with first-order kinetics. After intravenous administration, piprozoline was also rapidly transformed into metabolite I, as metabolite I existed in higher concentrations than the unchanged substance already after 6 min. In all cases, metabolite I was eliminated from the plasma with a half-life of approx. 2 h; 15-30% of the given dose appeared as metabolite I in urine, piprozoline itself could not be detected. After correction to uniform doses of 100 mg/kg, in all species the area under the plasma level curve of the metabolite amounted to approx. 12 mg/ml X h and the apparent volume of distribution to approx. 1-1.5 1/kg body weight. This indicates good availability in animal and man, regardless of the galenic formulation.