Currently available rat models for measuring gastric emptying are hampered by the necessity to kill the animals at the end of each experiment, which makes repetitive testing impossible. We have developed and validated a noninvasive test model, adapted from the 13C-octanoic breath test in humans, for repetitive measurements of gastric emptying in rats. Male Wistar rats were trained on a fixed protocol to eat a piece of pancake doped with 1 microg 13C-octanoic acid after 12 h fasting, and to stay thereafter in cylindrical glass cages. Breath tests were performed by a fully automated system of computer-guided switching valves, which collected consecutive breath samples. All breath samples were analysed by gas chromatography and isotope mass spectrometry. The area under the curve (AUC) from the cumulative 13CO2 excretion from 0 to 6 h was determined by the trapezium method to calculate the gastric half-emptying times (t(1/2)). Inter-day variability was determined. The effect of subcutaneous or intraperitoneal injection of saline was studied. The test was further validated for pharmacological interventions by oral administration of cisapride and parenteral administration of atropine, to induce, respectively. acceleration and delay of gastric emptying. Mean gastric emptying times +/- SD of 24 rats were 119.3 +/- 28.2 min, 138.7 +/- 26.0 min, and 124.5 +/- 30.9 min on three different test days. The mean coefficient of variation of three repeated measurements in the same 24 rats was 17.5%. No significant differences were observed after subcutaneous or intraperitoneal injection of saline. In a second test series of eight rats, cisapride significantly accelerated gastric emptying (mean t(1/2) 112.7 +/- 33.1 min, P < 0.05), while atropine caused a significant delay (mean t(1/2) 205.9 +/- 24.9 min, P < 0.05) when compared to control test results (mean t(1/2) 140.7 +/- 16.7 min) in the same rats. We validated the 13C-octanoic breath test to study gastric emptying in rats. This test method obviates the necessity to kill laboratory animals and allows repetitive measurements of gastric emptying to study its physiology or pathophysiology as well as the effect of pharmacological agents.