The possible synergism between caffeine and muscarinic antagonists to inhibit haloperidol-induced catalepsy was investigated with the bar test in rats. Pretreatment with low doses of caffeine (1-3 mg/kg), a non-selective adenosine antagonist, dose dependently reduced the intensity and increased the onset latency of catalepsy induced by haloperidol (0.5-2 mg/kg). Similar effects were produced by the muscarinic antagonists atropine (4.1 mg/kg), and trihexyphenidyl (THP, 0.01-3 mg/kg). THP inhibited catalepsy intensity with an ED(50) of 0.38 mg/kg, and increased its onset latency with an ED(50) of 0.52 mg/kg. The anticataleptic effect of anticholinergics was potentiated when a low dose of caffeine (1 mg/kg) was applied simultaneously. In the presence of caffeine, THP inhibited catalepsy intensity with an ED(50) of 0.19 mg/kg, and prolonged the latency with an ED(50) of 0.30 mg/kg. The synergism was more evident when THP was administered at subthreshold doses that were unable to modify haloperidol-induced catalepsy when applied alone, but produced a clear inhibition of catalepsy when injected with caffeine. To assess whether repeated administration of caffeine could induce tolerance to the synergism with THP, a group of rats was pretreated with three daily doses of caffeine (1 mg/kg) for seven days, and the catalepsy test was performed on the eighth day. In these animals, caffeine was still able to enhance the anticataleptic actions of THP, suggesting that repeated administration of 1 mg/kg caffeine does not induce tolerance to the synergism with anticholinergics. These results indicate that low doses of caffeine enhance the anticataleptic actions of muscarinic antagonists, and leave open the possibility of using caffeine as adjunctive therapy to reduce the doses and the adverse effects of anticholinergics in Parkinson's disease.