The transit of 99Tcm-labelled particles through the small intestine has been monitored in nine healthy subjects undergoing three levels of exercise. The mean ±1s.d. small intestine transit times under conditions of minimal, moderate and strenuous activity were 4.5±1.7 h, 5.4±2.5 h and 4.1±1.8 h, respectively. These findings indicate that the bioavailability of drugs from controlled release preparations passing through the small intestine is unlikely to be affected by variations in normal daily activity.