Carteolol, 5-(3-tert-butylamino-2-hydroxy) propoxy-3, 4-dihydrocarbostyril hydrochloride was found to be a beta-adrenergic partical agonist in the taenia caecum of guinea pig. Concentration response curves of carteolol and isoprenaline were parallelly shifted by propranolol (10^-7 M) suggesting that site of action of both the drugs was the same beta-adrenoceptor. The pD₂-values of carteolol obtained from the concentration action curves in mechanical response and increase of tissue concentration of cyclic AMP were significantly different from its pA₂-value against isoprenaline and its pK_i-value (negative log of its apparent dissociation constant). A possible explanation of our observation would seem to be as follows. Carteolol and isopranaline interact with the same beta-adrenoceptor where there may be two different sites : a binding site for agonist and binding site for antagonist.