1-Acetyl-4-phenyl-1,2,4-triazolidine-3,5-dione (APTD) has hypolipidaemic, anti-inflammatory, analgesic, antineoplastic, and aldose reductase inhibitory activities in animals. Disposition studies using pooled plasma and urine samples showed that [1-14C-acetyl]-1-acetyl-4-phenyl-1,2,4-triazolidine-3,5-dione (14C-APTD) had a maximum half-life of 20 hours. Urinary excretion accounted for less than 3% of the radioactivity elimination, while faecal excretion may account for up to 45% of the total elimination. In a 96-hour tissue distribution study, there was no sequestering of14C-APTD in any of the organs.14C-APTD demonstrated significant aqueous partitioning, and almost no binding to bovine serum albumin. In L1210tumour cells,14C-APTD was bound to DNA and RNA, and there was no binding to intracellular protein.14C-APTD underwent significant metabolism in mice. One metabolite excreted in urine was identified; two other possible metabolites were proposed.