1. The effects of catecholamines and some adrenoceptor agonists and antagonists on isolated preparations of human colonic smooth muscle obtained from surgical resections were examined. 2. Strips of circular smooth muscle displayed rhythmic myogenic spontaneous contractions which were inhibited by catecholamines with an order of potency of isoprenaline (1.0) > noradrenaline (0.32) > adrenaline (0.2). Phentolamine (0.7 microM) significantly shifted the noradrenaline concentration-response curve (CRC) to the right but had no significant effect on isoprenaline or adrenaline. Propranolol (1 microM) significantly shifted the isoprenaline to the right but had no significant effect on noradrenaline or adrenaline. 3. Salbutamol (30 microM) had no inhibitory effect on the spontaneous activity and ICI 118,551 (1 microM) had no effect on inhibitory responses to isoprenaline. Betaxolol (1 microM) significantly shifted the CRC to isoprenaline to the right. BRL 37344 had no effect on spontaneous activity. 4. Responsiveness of circular strips to catecholamines was not affected by age of the patient and no consistent differences between males and females were shown. 5. Strips of taenia coli exhibited little or no spontaneous phasic activity. Noradrenaline and isoprenaline relaxed KCl-induced tone. The effects of noradrenaline and isoprenaline were antagonized by propranolol but not by phentolamine. BRL 37344 had no effect on KCl-induced tone. 6. In conclusion, catecholamines relaxed spontaneous activity of human colon circular smooth muscle through an action on both alpha- and beta-adrenoceptors. The alpha-adrenoceptors were of the alpha 1-subtype. The beta-adrenoceptor-mediated relaxation appeared to be primarily beta 1. In taenia coli, catecholamines relaxed KCl-induced tone via beta-adrenoceptors only.