Organic mercury (CH3HgCl) with metal concentration 5 ppm in tap water was applied to rats suckling their newborn for the first 21 days of life. A second group of young rats took the mercury in their tap water 5 ppm from the 22nd to the 43rd day of postnatal life. Control rats drank tap water only. In 2-month-old male rats the following behavioral study was performed after saline or specific central dopamine receptor agonists and agonists apply (quinpirole, SKF-38393, haloperidol, SCH-23390): irritability, yawning behavior, oral activity, locomotion, exploratory activity, and catalepsy. In the striatum and frontal cortex of three examined groups the biogenic amines levels (DA, DOPAC, HVA, 3-MT, 5-HT, 5-HIAA, NA) were estimated by means of HPLC/ED technique, and DA and 5-HT turnover. The effect of quinpirole (a central dopamine D2 receptor agonists) was also examined on (3H)glucose uptake in discrete parts of the brain. It was shown that mercury affected behavioral changes after dopaminergic agents apply to adult animals when exposed in the period from the 22nd to 43rd day of postnatal development. Biochemical changes (biogenic amines level, turnover and (3H)glucose uptake) were more pronounced in adult animals exposed to mercury via mother's milk (1st to 21st day of life). In light of the above we conclude that early postnatal exposure of rats to organic mercury modulates activity of the central dopamine neurotransmitter system.