1. Plasma propranolol concentrations were measured in healthy old and young subjects following single oral doses of 40 mg, single i.v. infusions of 0.15 mg kg-1 and after nine 40 mg oral doses given four times daily. 2. In each of the three studies, the elderly had higher plasma propranolol concentrations than the young despite having similar apparent volumes of distribution. 3. The terminal half-life of propranolol was similar in the two groups after oral propranolol but significantly shorter in the young after intravenous dosing (P less than 0.05). 4. The bioavailability assessed from the concentration-time curves after i.v. and oral dosing was greater in the elderly (P less than 0.05). 5. The differences between peak concentrations observed in old and young subjects after single oral doses were maintained during chronic therapy and there was a correlation between the individual values obtained on multiple therapy with that after a single dose (P less than 0.05). 6. Ageing appears to affect the pharmacokinetics of propranolol in two ways. Firstly, distribution to the tissues appears to be slowed. Secondly, the increased bioavailability following oral administration suggests diminished intrinsic clearance by metabolism.