1. The possibility that prostacyclin (IP-) receptor agonists inhibit spontaneous contractions of the rat isolated colon by activating enteric neurones has been investigated. Cicaprost was used as the test agonist because of its high stability, selectivity and potency (IC50 = 3.8 nM). 2. The Na+ channel blockers saxitoxin (STX, 1 nM) and tetrodotoxin (TTX, 1 microM), whilst having little effect on resting spontaneous activity, virtually abolished the inhibitory actions of cicaprost (10 nM) and nicotine (3 microM); inhibitory responses to isoprenaline (20 nM) were not affected. Phentolamine (1 microM), propranolol (1 microM) and atropine (1 microM) had no effect on cicaprost inhibition. These data are compatible with release of inhibitory NANC transmitter(s) by cicaprost. 3. A transmitter role for nitric oxide was investigated. The nitric oxide synthase (NOS) inhibitor N omega-nitro-L-arginine methyl ester (L-NAME, 100 microM) inhibited the actions of both cicaprost (10 nM) and nicotine (3 microM) by 50-60%, but did not affect responses to isoprenaline (20 nM) or sodium nitroprusside (1-5 microM). The enantiomeric D-NAME (100 microM), which has negligible NOS inhibitory activity, had no effect on the action of cicaprost. 4. The involvement of purinergic transmitters was also investigated. Desensitization to the inhibitory action of ATP did not affect cicaprost responses. The P2x/P2y-receptor antagonist, suramin, at 300 microM blocked ATP responses, but not those due to adenosine; it did not affect cicaprost inhibition. The selective adenosine A1-receptor antagonist, DPCPX, used at a sufficiently high concentration (5 microM) to block adenosine A2-receptors, did not affect cicaprost inhibition. Apamin (25 nM), a blocker of calcium activated K+ channels on smooth muscle, abolished or markedly reduced the inhibitory actions of ATP and adenosine, and partially inhibited cicaprost and nicotine responses. The combination of L-NAME(100 microM) and apamin (25 nM) abolished cicaprost and nicotine responses.5. Investigation of vasoactive intestinal peptide (VIP) as a potential transmitter showed that its inhibitory action on the colon (IC50 = 50 nM) was partially inhibited by TTX (1 microM). alpha-Chymotrypsin abolished the effect of VIP but had no effect on cicaprost inhibition. Attempts to inhibit VIP responses using peptide antagonists and by agonist desensitization were unsuccessful.6. KCI (40 mM) contracted the colon and abolished spontaneous activity. Under these conditions,isoprenaline, sodium nitroprusside and ATP induced relaxation, whereas cicaprost (10-3 10 nM) had no effect. Cicaprost inhibited both the tone and the spontaneous activity induced by the EP1/EP3-receptor agonist, sulprostone (8.6 nM) but not when either TTX (1 microM) or KC1 (40 mM) was also present. On KCl-treated preparations, the prostacyclin analogue, iloprost (10-500 nM), induced contraction,presumably due to activation of EP-receptors.7. It is concluded that IP-receptor agonists inhibit the contractility of rat colon by stimulating the release of at least two transmitters from NANC enteric neurones. Nitric oxide appears to be one of the transmitters. The second transmitter mechanism is apamin-sensitive; the experimental results do not support ATP, adenosine or VIP as transmitter candidates. However, further studies using more potent and selective receptor antagonists are required.