1. Plasma drug concentrations after a single oral administration of a suspension of carbamazepine (CBZ) and a 20/200 CBZ Oros osmotic pump system were measured in eight healthy male volunteers. The oral suspension contained 100 mg CBZ labelled with the stable isotope nitrogen-15, whilst the Oros contained 200 mg unlabelled CBZ. Plasma concentrations of [15N]-CBZ and CBZ were measured simultaneously by gas chromatography-mass spectrometry. 2. The position of the CBZ Oros (labelled with indium-111) in the gastrointestinal tract was followed by gamma scintigraphy. Plasma drug concentrations after the two treatments were used to relate pharmacokinetic with transit data. 3. The Oros was taken after breakfast and gastric emptying occurred between 1.1- greater than h post-dosing (median, 5.3 h). Small intestinal transit times ranged from 1.5- greater than 3.6 h, with a median of 2.2 h. There were wide individual variations in colonic transit, and the total transit time ranged from 10-60 h (median, 22 h). 4. Relative systemic bioavailability of CBZ from the Oros was reduced compared with that from the suspension (mean dose normalised AUC ratio = 0.69 +/- 0.17; mean dose-normalised AUC ratio = 0.85 +/- 0.13, allowing for actual release from the Oros system). 5. The in vivo absorption of drug into the systemic circulation from the Oros was estimated using the Wagner-Nelson method. This showed that absorption of CBZ was rapid when the Oros was present in the stomach and small intestine, the rate being determined by the release of drug from the system.(ABSTRACT TRUNCATED AT 250 WORDS)