The efficacy of 1-(2,6-dimethylphenyl)-3-methyl-amidinourea hydrochloride (lidamidine HCl, WHR-1142 A) an aryl-substituted amidinourea recently synthesized, was compared with that of loperamide in 32 patients with acute diarrhoea. The results of the study show that lidamidine HCl and loperamide had comparable effects in the pharmacological treatment of acute non-specific diarrhoea. Lidamidine HCl was also shown to be well tolerated; side-effects were generally minor and self-limiting.