The effects of GABA and 5-HT on the electric activities of neurons of nucleus parafascicularis thalami (Pf) were observed in anesthetized rats with multimicropipete and iontophoresis and technique. It was found that: (1) Pain discharges of the Pf neurons were inhibited by microiontophoretically applied (MIA) GABA, an effect that could be reversed by MIA picrotoxin. Picrotoxin alone could enhance the pain discharges. The results indicate that GABA may be an inhibitory neurotransmitter responsible for the electric activities of Pf neurons. (2) MIA 5-HT has both excitatory and inhibitory effects on the pain discharge of Pf neurons, while only the former could be blocked by MIA cyproheptadine. It was suggested that 5-HT may participate in the modulation of the electric activities of the Pf neurons with different subtypes of receptors.