Substance P, somatostatin, enkephalin and vasoactive intestinal polypeptide (VIP) did not mimic the inhibitory responses to non-adrenergic, non-cholinergic nerve stimulation. Substance P (0.1-10 microgram/ml) always caused contraction, enkephalin (0.1-10 microgram/ml) and somatostatin (0.1 microgram/ml) were inactive, while VIP (0.1-1 microgram/ml) produced very slow relaxation, taking about 4 min to reach a maximum after a latency of about 60 sec. Low concentrations of neurotensin (1-10 ng/mg) caused contraction, but at higher concentrations (50-1000 ng/ml) it produced a biphasic response which consisted of an initial contraction followed by a slow relaxation. In high tone preparations, the slow relaxation did not mimic the nerve-mediated response, taking approximately 43 sec. to reach maximum, after a long latency of about 15 sec. In contrast, ATP (0.1-50 microgram/ml) mimicked closely the rapid responses to non-adrenergic, non-cholinergic nerve stimulation in all preparations, whether the tone was low, medium or high. The time for the inhibitory response to reach maximum was about 15 sec after a latency of approximately 1 sec. Indomethacin (3.4-34 microgram/ml) did not unmask any inhibitory responses to any of the peptides. It is concluded that ATP remains the most likely substance to be the inhibitory transmitter released from non-adrenergic, non-cholinergic nerves supplying the smooth muscle of the taenia coli.