The influence of the site of drug delivery on the systemic availability of metoprolol has been evaluated by measuring plasma drug concentrations in six healthy volunteers after administration of a continuous 13.5 h intragastric infusion and a 14/190 Oros controlled-release dosage form, on two separate occasions. The same total amount of drug was administered at the same rate on both occasions but the Oros system moved through the gut whereas the site of the infusion was constant. The differences between treatments were confined largely to the period 6-15 h after dosing when lower plasma concentrations were obtained after administration of the Oros system. The levels after 20 h were higher for Oros, however, reflecting its longer duration of drug release. The amount of drug reaching the circulation was 19.8% less for the Oros preparation compared with intragastric infusion but this was not due to incomplete release since the residual amounts of drug in three systems recovered from faeces corresponded to less than 12% of the administered dose. Analysis of the plasma profiles by the Wagner-Nelson method indicated a reasonable agreement between in vitro release and in vivo absorption. The appearance of drug in plasma was delayed for both treatments, and for Oros the apparent absorption rate slowed 6 h after dosing. Plasma profiles after 14/190 metoprolol Oros were consistent with prolonged in vivo delivery and absorption from the gut. The absorption process, however, was associated with some reduction both in the rate, after 6 h, and in the total amount reaching the circulation.