The bioavailability of amprolium (APL) was measured after intravenous (i.v.) and oral (p.o.) administration to chickens. Twelve healthy chickens weighing 1.28–1.41 kg received a dose of 13 mg APL/kg intravenously, and 13 or 26 mg APL/kg orally in both a fasted and a nonfasted condition in a Latin square design. Plasma samples were taken from the subwing vein for determination of APL concentration by HPLC method. The data following intravenous and oral administration were best fitted by 2-compartment and 1-compartment models, respectively, using weighted nonlinear least squares regression. The half-life beta t½β, volume of distribution (Vd) and total body clearance (Cl) after intravenous administration were 0.21 h, 0.12 L/kg and 1.32 L/h.kg, respectively. The elimination half-life (t½ Kel) after oral administration was 0.292–0.654 h which is 1.5–3.2 times longer than after intravenous administration, suggesting the presence of a ‘flip-flop’ phenomenon in chickens. The maximum plasma concentration (Cmax) of 13 mg/kg APL administered orally to chickens during fasting was significantly (about four times) higher than that during nonfasting (P < 0.05). Bioavailability during nonfasting was from 2.3 to 2.6%, and 6.4% during fasting.