The contractile response of intestinal strips in rainbow trout (Oncorhynchus mykiss Walbaum, 1792) to the administration of histamine was assessed by means of the organ bath technique. Intestinal strips were isolated from 16 clinically healthy fish and mounted in organ baths. Histamine was compared with the full agonist serotonin, to evaluate their contractile efficacy and potency. Serotonin elicited a concentration-related contraction in all examined intestinal strips, whereas histamine induced the contraction only in 14 exemplars. Of these, seven exhibited a concentration-related response. A sigmoidal curve was fitted from data (R(2) = 0.55) and its best fit values were compared with those of serotonin. Interestingly, histamine exhibited the same efficacy (E(max)) as serotonin (F-test, p > 0.05), but showed lower potency (by an order of magnitude) (F-test, p < 0.01). Moreover, the effect of the H1 antagonist, pyrilamine, has been tested to exclude aspecific contraction due to other agonists eventually released in situ following histamine administration. Pyrilamine showed a marked concentration-related antagonist action on the contractility induced by histamine with complete contractility antagonism at 10(-4)M. The authors suggest that the responses to histamine measured in the present study reflect a less sensitive response to an exogenous source of histamine, possibly due to bacterial metabolism.