The aim of this study was to evaluate the action of electroacupuncture (EA) on the activity of serotonergic neuron in developing mammalian brain regions. A single exposure of developing rats (3-12 weeks) to electroacupuneture (20-60 Hz, 1 Volt, for 2-10 min) show that very young rats (3 weeks) required higher frequency (60 Hz, 1 Volt) and more time (10 min) to develop electroacupuncture-induced analgesia (EA.A) than that observed in the young (6 weeks) (40 Hz, 1 Volt, for 5 min) and mature (12 weeks) (20 Hz, I Volt, for 2 min) rats. The study of the serotonergie system by measuring the steady state levels of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIA.A) and pargyline-induced accumulation of 5-HT and efflux of 5-HIA.A in different brain regions e.g. cerebral cortex (Cx), corpus striatum (Cs), cerebellum (Cb) thalamus (Th) and hypothalamus (H) of developing rat exposed to EA with corresponding effective dose and time as mentioned above, showed that serotonergic activity was increased in Cb and Th of developing rats (3 weeks to 3 months) with the development of EA (60-20 Hz, 1 Volt, for 10 min) -induced analgesia. This result, thus, suggest that (a) EAA is age dependent and Co) serotonergie system in Cb and Th regions of developing brain in 3-12 weeks of age is activated with EAA..