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The pharmacokinetics, metabolism, excretion and tissue residues of phenylbutazone (PBZ) in the horse were studied following both intravenous and oral administration of the drug at a dose rate of 4.4 mg/kg. A 72-hour blood sampling schedule failed to demonstrate a third exponential phase; the plasma disposition following intravenous injection being described by a two compartment open model, with the following elimination phase parameters: beta = 0.13h-1, t1/2 beta = 5.46h, Vdarea = 0.141 1/kg and C1B = 17.9 ml/kg/h. The hydroxylated metabolites oxyphenbutazone (OPBZ) and gamma-hydroxyphenylbutazone (OHPBZ) were present in detectable concentrations in plasma for 72 and 24 h, respectively. After 36 h OPBZ concentrations exceeded plasma PBZ concentrations. In urine the principal metabolites were OPBZ and OHPBZ but smaller concentrations of another compound, probably gamma-hydroxyoxyphenbutazone (OHOPBZ), were also detected. The percentages of the administered dose recovered from urine were 30.7, 39.0 and 40.3 after 24, 48 and 72 h from the time of injection. Recovery of PBZ and its metabolites from urine was significantly reduced in the first 24 h after oral dosing when the horses had free access to hay, probably as a result of markedly delayed absorption, but this did not occur in animals deprived of food for a few hours before and after dosing. Determination of approximate values of urine/plasma (U/P) concentration ratios for PBZ and its metabolites relative to endogenous creatinine U/P concentration ratio suggested that PBZ was filtered in small amounts only because of the high degree of plasma protein binding and then excreted by diffusion trapping in the alkaline urine. Much higher U/P ratios were obtained for the hydroxylated derivatives, and one at least (OHPBZ) was secreted into urine.