We have found that after intraperitoneal injecting atropine the analgesic effect of both stimulating Sm I cortex and acupuncture were decreased. It suggests that acetylcholine (ACh) may be involved in the corticofugal modulation of pain. In order to elucidate this idea the effects of stimulating Sm I cortex on tail flick latency (TFL) after ventrical microinjecting atropine were investigated. The experiments were carried out on Wistar rats. Under the state of anaesthesia a pair of silver ball electrodes were put on the dura for electrical stimulating Sm I and the ipsilateral ventricle was cannulated for microinjecting atropine. Recordings were made 5-6 hours or 24 hours after operation. Immediately after cessation of stimulating SmI and 1', 2', 3.5', 5', 7.5', 10' and 15' later TFL were measured consecutively. The results were as follows: 1. The mean value of TFL was 2.44 +/- 0.11 sec (n = 20). It was prolonged (p < 0.05) within 0'-5' after stimulation of SmI. Therefore, it indicated that analgesic effects were produced by stimulating SmI cortex. 2. It was found that under the background of microinjecting atropine (10 micrograms/2 microliters) TFL remained unchanged after stimulating SmI cortex (n = 10, P > 0.05). There was a significant difference between the two treatments: simple stimulating SmI and atropine plus stimulating SmI at the same animals in TFL (P < 0.05) within 0' and 1' after cessation of stimulating SmI. It indicated that the analgesic effects of stimulating SmI cortex were decreased by blocking M-receptors. 3. Under the background of microinjecting normal saline.(ABSTRACT TRUNCATED AT 250 WORDS)