text: Labetalol (> or = 3 x 10(-8) M) and dilevalol (> or = 10(-8) M) caused surmountable antagonism of the << isoprenaline >> responses of the atria and the pA2 values were 8.60 and 8.98 at the [[ beta 1-adrenoceptors ]] of the rat left atria and 7.90 and 8.31, respectively, on the guinea-pig left atria which has functional beta 1- and beta 2-adrenoceptors.