1 Contractile responses of canine intestinal circular and longitudinal muscles to field stimulation (20 Hz, 1 ms, 30 V/cm, for 5 s) were inhibited by treatment with atropine (0.1 microgram/ml), indicating that the response to field stimulation was mediated by acetylcholine (ACh). 2 Prostaglandins E1 (PGE1), PGE2 and PGF2 alpha inhibited the response of circular but not longitudinal muscle to field stimulation, although PGF2 alpha was less effective than PGE1 and E2. 3 PGE1 was much less active in inhibiting the response of circular muscle to ACh than to field stimulation, suggesting that prostaglandins might act predominantly at prejunctional sites to prevent the release of ACh. 4 Indomethacin (1 microgram/ml) potentiated the response of circular muscle but not longitudinal muscle to field stimulation. 5 Release of PGE-like compounds from circular muscle only, was increased by field stimulation at 20 Hz (total of 1000 pulses) and ACh (10 micrograms/ml), but not by a lower frequency (2 Hz, total of 2400 pulses) which produced only a slight contraction. This finding may indicate that prostaglandins were released predominantly from the muscle. 6 Prostaglandins may exert a negative feedback mechanism of excitatory transmission in circular muscle but not in longitudinal muscle of canine small intestine.