twenty one adult horses, males and females, were pretreated with 0.14 mg/kg of hyoscine-n-butylbromide intravenously and injected 5 minutes later with 0.02 mg/kg, iv of detomidine (group a, n = 9) or saline (group b, n = 12). mean arterial pressure measurements and electrocardiography were performed during 65 minutes. after hyoscine injection the heart rate was increased by 43% and 65% in a and b groups, respectively. heart rate remained increased after injection of detomidine, returning to baseline values after 15 minutes. no increase in the mean arterial pressure (map) was noticed after hyoscine but the map was increased by 62% after detomidine, returning to basal measurements until the end of observation time. no additional increase was noted in the group b horses. hyoscine shortened pr and qt intervals in both groups, but after detomidine, pr and qt intervals enlarged significantly at the end of the experiment. the second degree atrioventricular block occurred in 3 horses after 40 minutes only in group b. it was concluded that hyoscine prevented detomidine induced bradycardia and may be an useful drug combination against the bradycardia induced by this alpha-2 agonist, in horses.