The tissue distribution of propranolol after i.v. administration (1.5 and 7.5 mg/kg) was studied in rats. Lung, brain and kidney showed extensive propranolol tissue binding. Propranolol uptake by lung seemed to be a saturable process. In contrast to the above tissues, liver propranolol concentrations remained low over the time period of study. An increase in the propranolol T1/2 was noted at the high dose and seemed to result from reduced systemic clearance and an increase in the apparent volume of distribution. Microsomal and mitochondrial fractions from several tissues contained substantial amounts of propranolol after i.v. administration and in vitro incubation with homogenates. Cytosol proteins did not bind significant amounts of propranolol. Equilibrium dialysis studies with rat liver mitochondrial and microsomal fractions revealed both high affinity, low capacity propranolol binding sites and low affinity, high capacity sites. At low concentrations, propranolol interaction with rat liver microsomes produced a type I difference spectrum with high affinity binding of similar magnitude to that observed with equilibrium dialysis. Higher concentrations of propranolol produced a saturable shift in the difference spectra with reduced binding affinity for propranolol. Results from these studies indicate that particulate fractions from several tissues contribute to the extensive tissue binding of propranolol.