The bioavailability of isbufylline was assessed in male rabbits given 12 mg/kg i.v. (intravenous) or per os (oral) according to a randomized design. The concentrations of unbound (fu = 54.0) isbufylline were considered in plasma as a function of time, after i.v. and oral administration. After oral administration in saline solution, a mean absolute oral bioavailability of 59.6% was calculated. The drug is rapidly absorbed and the comparison of the kinetic profiles after i.v. and per os administration, revealed a rapid elimination: t1/2 27.3 and 28.8 min respectively, and a total body clearance of 67.06 ml/min/kg. Urinary recovery 0-48 h accounted for less than 1% of the dose.