Six mature Holstein bulls were each given 10 mg of phenylbutazone (PBZ)/kg of body weight, PO. Of the 6 bulls, 3 were given 10 mg of PBZ/kg by rapid IV administration 4 weeks later. Plasma concentration-vs-time data were analyzed, using nonlinear regression modeling (sum of exponential functions). The harmonic mean of the biologic half-life of PBZ was 62.6 +/- 12.9 hours after oral administration and 61.6 +/- 7.2 hours after IV administration. The mean residence time was 94.61 +/- 8.44 hours and 90.49 +/- 8.93 hours for oral and IV administration, respectively. The mean total body clearance was 0.0015 +/- 0.0003 L/h/kg, with the mean apparent volume of distribution 0.134 +/- 0.021 L/kg. Mean bioavailability was 73 +/- 2% after oral administration. Phenylbutazone was adequately absorbed from the gastrointestinal tract in bulls. The apparent volume of distribution was small, indicating that PBZ distributed mainly into plasma and extracellular fluid. The total body clearance was also small, which accounted for the long half-life of PBZ in bulls.

Six mature Holstein bulls were given an 8-day course of phenylbutazone (PBZ) orally (loading dose, 12 mg of PBZ/kg of body weight and 7 maintenance doses of 6 mg of PBZ/kg, q 24 h). Plasma concentration-vs-time data were analyzed, using nonlinear regression modeling. The harmonic mean +/- pseudo-SD of the biologic half-life of PBZ was 61.8 +/- 12.8 hours. The arithmetic mean +/- SEM of the total body clearance and apparent volume of distribution were 0.0021 +/- 0.0001 L/h/kg and 0.201 +/- 0.009 L/kg, respectively. The predicted mean minimal plasma concentration of PBZ with this dosage regimen was 75.06 +/- 4.05 microgram/ml. The predicted minimal plasma drug concentration was compared with the observed minimal plasma drug concentration in another group of bulls treated with PBZ for at least 60 days. Sixteen mature Holstein bulls were given approximately 6 mg of PBZ/kg, PO, daily for various musculoskeletal disorders. The mean observed minimal plasma concentration of PBZ in the 16 bulls was 76.10 +/- 2.04 microgram/ml, whereas the mean predicted minimal plasma concentration was 74.69 +/- 3.10 microgram/ml. Dosages of 4 to 6 mg of PBZ/kg, q 24 h, or 10 to 14 mg of PBZ/kg, q 48 h, provided therapeutic plasma concentrations of PBZ with minimal steady-state concentrations between 50 and 70 microgram/ml.