Male Sprague-Dawley rats maintained under controlled environmental conditions were used. Choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) activities were determined in the cerebral cortex, bulbus olfactorius, midbrain, hypothalamus, hippocampus, cerebellum, pons and medulla oblongata of control rats and rats treated with apomorphine (15 mg/kg, i.p.) after a single dose or after a second dose administered 24 h later at 10.00 or 22.00 h. Results of this experiment indicate that the repeated administration of apomorphine at 10.00 h was associated with rapid development of tolerance to the hypothermic action of this drug. A single injection of apomorphine resulted in significant (p less than 0.01) decrease of AChE and significant increase of ChAT activity (p less than 0.01) in the cortex, hypothalamus and pons. There were no significant differences between tolerant and control animals in the activities of AChE or ChAT in the bulbus olfactorius, cerebral cortex, midbrain, pons or medulla. On the other hand, repeated administration of apomorphine at 22.00 h was not associated with tolerance to the hypothermic action of this drug. A single injection of apomorphine at 22.00 h resulted in significant (p less than 0.01) increase in AChE activity of the midbrain, hippocampus, and the medulla oblongata with no significant changes in the cerebral cortex and bulbus olfactorius. There was a significant decline (p less than 0.01) of ChAT activity of the hypothalamus, hippocampus, cerebral cortex and the medulla oblongata (p less than 0.05). Meanwhile, there was a significant (p less than 0.05) increase of ChAT activity of the midbrain with no significant changes in the cerebellum, pons and the bulbus olfactorius.(ABSTRACT TRUNCATED AT 250 WORDS)