This study reports a method for the evaluation of both gastric emptying and gastrocecal transit times in rats simultaneously by using the same breath testing system measuring equipment. Male rats were used after fasting. Gastric emptying and gastrocecal transit time were evaluated by using [1-¹³C] acetic acid (8 mg/kg) and lactose-[¹³C] ureide (60 mg/kg), respectively. A mixture of both ¹³C-labelled compounds dissolved in Racol (liquid nutrient formula) was administered orally. The level of ¹³CO₂ in the expired air was measured using an infrared spectrometer at appropriate intervals for a period of 420 min. The level of ¹³CO₂ in the expired air from [1-¹³C] acetic acid increased with time and peaked at about 30 min before decreasing, while that from lactose-[¹³C] ureide increased after about 180 min. The time taken to reach the maximum value of gastric emptying (Tmax) was 27.5 ± 0.9 min. Gastrocecal transit time was 180 ± 11.5 min, which was calculated as the time before the ¹³CO₂ value increased again. These results accorded with the results of gastric emptying and gastrocecal transit time evaluated by using each ¹³C-labelled compound separately. These results demonstrate that this method is useful for the simultaneous evaluation of gastric emptying and gastrocecal transit times in rats.