This study describes the spasmolytic, antidiarrhoeal, antisecretory, bronchodilatory and urinary bladder relaxant properties of Hyoscyamus niger to rationalize some of its medicinal uses. The crude extract of H. niger seeds (Hn.Cr) caused a complete concentration-dependent relaxation of spontaneous contractions of rabbit jejunum, similar to that caused by verapamil, whereas atropine produced partial inhibition. Hn.Cr inhibited contractions induced by carbachol (1 microM) and K(+) (80 mM) in a pattern similar to that of dicyclomine, but different from verapamil and atropine. Hn.Cr shifted the Ca(2+) concentration-response curves to the right, similar to that caused by verapamil and dicyclomine, suggesting a Ca(2+) channel-blocking mechanism in addition to an anticholinergic effect. In the guinea-pig ileum, Hn.Cr produced a rightward parallel shift of the acetylcholine curves, followed by a non-parallel shift with suppression of the maximum response at a higher concentration, similar to that caused by dicyclomine, but different from that of verapamil and atropine. Hn.Cr exhibited antidiarrhoeal and antisecretory effects against castor oil-induced diarrhoea and intestinal fluid accumulation in mice. In guinea-pig trachea and rabbit urinary bladder tissues, Hn.Cr caused relaxation of carbachol (1 microM) and K(+) (80 mM) induced contractions at around 10 and 25 times lower concentrations than in gut, respectively, and shifted carbachol curves to the right. Only the organic fractions of the extract had a Ca(2+) antagonist effect, whereas both organic and aqueous fractions had anticholinergic effect. A constituent, beta-sitosterol exhibited Ca(2+) channel-blocking action. These results suggest that the antispasmodic effect of H. niger is mediated through a combination of anticholinergic and Ca(2+) antagonist mechanisms. The relaxant effects of Hn.Cr occur at much lower concentrations in the trachea and bladder. This study offers explanations for the medicinal use of H. niger in treating gastrointestinal and respiratory disorders and bladder hyperactivity.