The relative lack of specificity of atropine as a competitive antagonist of muscarinic receptors is a frequent cause of undesirable parasympathetic side effects. Consequently, new tropane alkaloids with potentially greater selectivity are usually seen with real interest. The cholinergic antagonistic effects of a purified mixture of tropane alkaloids extracted from Schizanthus hookeri were evaluated in rat ileum. For this purpose, ileal segments were obtained from randomly selected male Sprague-Dawley rats, and the effect of 1 . 10(-4), 1 . 10(-3), and 1 . 10(-2) mg/mL of the purified mixture of alkaloids on the contractile response of the ileum induced with increasing doses of carbachol (5 . 10(-8)-8 . 10(-4) M) was determined. The results were compared with those obtained in the presence of 3.46 . 10(-7), 3.46 . 10(-6), and 3.46 . 10(-5) mg/mL atropine as an agonist. Tropane alkaloids extracted from Schizanthus hookeri competitively antagonized acetylcholine muscarinic receptors.