Two varieties of neurons were found in nucleus parafascicularis (pf) of the rat: one responds to noxious stimuli with an increase in firing (pain-excited neuron, PEN), the other with a decrease in firing (pain-inhibited neuron, PIN). Electroacupuncture (EA) has been shown to suppress PEN and excite PIN, which can be taken as an electrophysiological index for EA analgesia. This effect of EA subsided after prolonged (6 h) EA stimulation, suggesting the development of tolerance to EA. Intracerebroventricular (icv) injection of CCK-8 antiserum aiming at neutralizing endogenously released CCK-8 resulted in a complete restoration of the EA effect. Normal rabbit serum was not effective. CCK-8 antiserum per se did not affect the firing pattern of the PEN or PIN in nontolerant rat. The results obtained from single neuron recording in anesthetized animals thus confirmed those obtained in intact animals using the tail flick as the end point, implying that an excess of endogenously released CCK-8 may constitute one of the mechanisms for the development of EA tolerance.