The disposition of dl-propranolol was studied in spontaneously hypertensive rats (SHR), both after subcutaneous (s.c.) and intracerebroventricular (i.c.v.) injection of 1 mg/kg. 1. Upon s.c. injection propranolol appeared rapidly in plasma. A maximum concentration of 374 +/- 33 ng/ml (N = 10) was reached 5 min after injection. After a distribution phase with a half-life of t 1/2 alpha = 17 min propranolol was eliminated with a t 1/2 beta = 59 min. 2. Both propranolol and its metabolites were taken up rapidly into all tissues studied. Highest concentrations (10.4 +/- 1.5 micrograms/g, N = 5) were found in lungs 30 min after injection. 3. Neither propranolol nor its metabolites accumulated in any of the tissues examined. 4. Upon i.c.v. injection of propranolol, a maximal concentration of 573 +/- 47 ng/ml (N = 3) was reached in plasma already 2 min after injection. In this case t 1/2 alpha was 13 min and t 1/2 beta was 80 min. 5. Dialysis experiments indicated that propranolol is bound to plasma proteins for 92% in the concentration range of 20--100 ng/ml. With increasing concentrations binding diminishes progressively. At the highest concentration tested (345 ng/ml) only 76% was bound. It is concluded that s.c. and i.c.v. injection of an identical dose of propranolol gives a similar plasma concentration-time profile. Moreover, it is suggested that the pharmacokinetic behaviour of propranolol in SHR does not explain the delayed antihypertensive effect of this drug.