High magnesium (Mg) diets induce diarrhea in chicks. Addition of 0.3, 0.5 or 0.9% Mg (as MgO) to a corn-soy basal diet caused a dose-related lower gut passage time (GPT) (the first appearance of colored markers from feed in excreta), with the threshold for catharsis being approximately the 0.3% addition. Supplementation of the high Mg diets with a water-absorbent clay did not change the chicks' GPT. The beta-adrenergic blocking agent propranolol (1.68 mg/kg body wt) or the alpha-adrenergic blocking agent phenoxybenzamine (0.28 mg/kg body wt) did not effect the GPT of chicks fed high Mg diets. However, both the muscarinic blocker methscopolamine (0.12 mg/kg body wt) and the serotonergic blocker methysergide (0.2 mg/kg body wt) resulted in a higher GPT of chicks fed high Mg diets compared to corn-soy fed control chicks. Analysis of the osmotic load of excreta or intestinal contents of high Mg-fed chicks showed no differences from osmotic loads of control-fed chicks. When the osmotic loads observed were partitioned into mineral cationic and anionic components, the Mg content was found to be higher in intestines of chicks, but this higher Mg was paralleled and/or surpassed by chloride (Cl) content. From these data the authors conclude that Mg-induced cartharsis is due not simply to the hyperosmotic effect of Mg per se but to neural or endocrine arcs that affect the secretion or absorption of Cl as well.