1. Experiments were performed to investigate the possible interaction between tertatolol and the "atypical" beta-adrenoceptor present in guinea-pig ileum. 2. In the electrically field stimulated guinea-pig ileum, both isoprenaline and BRL-37344 produce concentration-dependent inhibition of the "twitch" response. 3. Schild regression analysis of the interaction between isoprenaline and tertatolol indicated two sites of action and yielded an apparent pA2 value for tertatolol of 6.8 at the "atypical" beta-adrenoceptor. 4. BRL-37344 selectively activated the "atypical" beta-adrenoceptor and tertatolol again yielded an apparent pA2 value of 6.7 and 6.8.