Effects of senna on the myenteric plexus of the colon were investigated in view of earlier reports that this anthraquinone cathartic depletes the plexus of its intrinsic neurons. Rats and mice were given purgative doses of sennosides in their drinking water for 4 and 5 months, respectively. Body growth was reduced, and the weight of the colon with its contents was increased relative to the weight of the whole body in the treated animals. The latter change was attributed to depressed propulsive motility of the large intestine. Total numbers of myenteric neurons were determined from whole-mount preparations stained with Cuprolinic Blue-magnesium chloride, which selectively coloured the neuronal somata. The number of neurons in the rat's colon was unaffected by treatment with senna, but the colons of the treated mice contained significantly more neurons than those of their controls. Staining with antisera to 10 putative neurotransmitters or their associated enzymes revealed immunoreactive somata and axons in the myenteric plexus. Treatment with senna was not associated with absence of neuronal somata or fibres stainable with any of the antisera in either species. Thus, there was no evidence of toxic destruction of any identifiable population of neurons that might have been too small to affect the total counts. We conclude that senna does not kill myenteric neurons in the colon of the rat or mouse.