Quantity and consistency of the faecal output, large intestine transit time, and colonic net fluid absorption were investigated in rats after oral administration of sennosides A + B (12.5-200 mg kg-1). The release of normal faecal pellets was accelerated 3-4 h after drug administration; excretion of soft faeces was evident within 4-5 h and reached its maximum 5-7 h after administration. Large intestine transit time was dose- and time-dependently influenced by sennoside treatment. A highly significant reduction in transit time from more than 6 h in controls to 90 min for a 2 h pretreatment and a nearly maximal reduction to 30 min for a 4 h pretreatment was induced by a dose of 50 mg kg-1. Inhibition of net fluid absorption in the colon was maximal with the same dose, but clearly more pronounced after a 6 h pretreatment period than after a 4 h period. Since the increase in fluid volume due to net fluid secretion is delayed compared with the acceleration of large intestine transit, the early motility effect seems to be largely independent of the changes in absorption mechanisms. Therefore, the laxative effect of the sennosides consists of changes in colon motility as well as in colonic fluid absorption, but motility may be an earlier and more sensitive parameter than net absorption.