In experiments on guinea-pig isolated tracheal spirals, clonidine, in concentrations of 10(-6) to 3 X 10(-4) M, had a contracting effect which was strongly inhibited by prazosin but not significantly modified by yohimbine. Moreover, clonidine (3 X 10(-5) to 3 X 10(-4) M) potentiated histamine-induced contractions; this latter effect was inhibited specifically by alpha 1-adrenoceptor antagonists (e.g. prazosin, AR-C 239) but unmodified by yohimbine, nicardipine or agents acting on the arachidonic acid cascade. It would therefore appear that clonidine in high concentrations contracts the guinea-pig trachea by stimulating alpha 1-adrenoceptors and that, contrary to what has been reported with other animal species, notably the dog, the guinea-pig trachea is devoid of alpha 2-adrenoceptors that mediate contractions.