The actions of alkylating pindolol (N8-bromoacetyl-N1-3′-(4-indolyloxy)-2′-hydroxypropyl-[z]-1,8-diamino-p-menthane; BIM) have been examined on β-adrenoceptors in guinea-pig left atria and trachea. In organ bath experiments, addition of BIM (≥0.1μM), followed by washout, produced concentration-dependent rightward shifts of the dose-response curve to cumulative additions of (-)-isoprenaline and oxymethylene-isoprenaline and reductions in the maximal response. The "apparent" pA2; value for BIM was 9.23 ± 0.20 (slope = 0.98 ± 0.20). Changes in the maximal density of β-adrenoceptor binding sites were determined in guinea-pig left atrial membranes using [125I]-cyanopindolol. BIM (0.1, 1.0 and 10 μM) produced 14, 23 and 41% reductions in Bmax with no change in KD. The binding of [125I]-BIM to guinea-pig left atrial membranes had a high non-specific binding component and a pseudo-Hill coefficient less than unity. The "apparent" KD, value of [125I]-BIM at β-adrenoceptor binding sites was 85.7 ± 57.9 pM.