Relaxation of carbachol pre-contracted human colonic muscle to (–)-isoprenaline was examined in circular, longitudinal and taenia coli preparations to determine the β-adrenoceptor subtypes involved. β1-, β2- and β3-Adrenoceptor mRNAs were also measured in colonic muscle and mucosa.(–)-Isoprenaline caused relaxation of longitudinal smooth muscle preparations with pEC50=7.39±0.12, and this response was inhibited by both propranolol (0.1 μM, pKB 8.55±0.12) and the selective β1-antagonist, CGP 20712A (0.1 μM, pKB 8.80±0.20), while the selective β2-antagonist, ICI 118551 (0.1 μM) failed to inhibit isoprenaline relaxation consistently.(–)-Isoprenaline caused relaxation of taenia coli with a pEC50 of 6.70±0.17. Propranolol (0.1 μM), CGP 20712A (0.1 μM) and ICI 118551 (0.1 μM) inhibited the isoprenaline response with similar low affinities (pKB values 7.93, 7.71 and 7.54, respectively). Carbachol pre-contracted circular smooth muscle preparations failed to relax consistently to isoprenaline and these responses were not characterized.β1- and β2-Adrenoceptor mRNAs were present in circular/longitudinal muscle samples and taenia coli samples, and lower levels were detected in mucosa. β3-mRNA was also present in both muscle preparations but was not detected in human colonic mucosa.In summary, β1-adrenoceptors are the predominant subtype mediating isoprenaline-induced relaxation of the thin longitudinal smooth muscle of human colon, while β3-receptors do not appear to be involved in these responses. However, β3-adrenoceptors may play a role in relaxation of the taenia coli as conventional antagonist affinities are low. β3-Adrenoceptor mRNA was present in taenia coli and circular/longitudinal smooth muscle but absent from human colonic mucosa.