The in vitro metabolism of generally labelled 3H-3-(1-hydroxy-2-piperidinoethyl)-5-phenylisoxazole (3H-31252-S) using the 9000xg supernatant fraction of rabbit liver homogenate was studied. The metabolites were indentified and determined quantitatively by reverse isotope dilution analysis. The results indicated that it was extensively metabolized and the main metabolic pathways were N-oxidation of piperidine and p-hydroxylation of phenyl ring. 3-1-Hydroxy-2-(4-hydroxypiperidino) ethyl}-5-phenylisoxazole (XVII) showed slightly stronger analgesic activity than the parent compound 31252-S. All metabolites were less foxic than the parent 31252-S.