The uptake of tyrosine as precursor of dopamine in striatal nerve endings was studied in rat striatal synaptosomes to evaluate the role of different bilirubin concentrations on this mechanism of synaptic neurotransmission. Freshly prepared striatal synaptosomes obtained from adult male Sprague-Dawley rats (200-300 g) were homogenized in about 25 vol of 0.32 M sucrose and 5 nM Hepes at pH 7.4 and centrifugated at 12,000-16,000 rpm for 20 min. 14C tyrosine was evaporated under N2 and resuspended in tyrosine 100 microliters/ml. Different concentrations of crystalline bilirubin ranging from 70 to 140 microM at 10-min intervals were prepared. Eighty microliters of synaptosomes were added to 5 microliters of the different bilirubin concentrations and 20 microliters of 14C tyrosine. The results of this study show a statistically significant correlation between bilirubin levels and tyrosine uptake in rat synaptosomes (p < 0.05) supporting the hypothesis that the effect of bilirubin on neuronal excitability is dose-dependent.