The mechanisms by which bilirubin causes neurotoxicity in newborns have not been well defined, but an involvement in synaptic transmission appears possible. Herein we present evidence for an inhibitory effect of bilirubin on both basal and depolarization-induced (50 mM KCl) phosphorylation of synapsin I, a synaptic vesicle-associated protein that may play a role in neurotransmitter release. Synaptosomes from rat cerebral cortices, prelabeled with 32P in vitro to label the intraterminal ATP pool, were incubated with or without bilirubin and bovine serum albumin (added as a stabilizer) at varying doses and for different time intervals. Some preparations were also depolarized by high KCl concentrations to induce Ca++ influx. The phosphorylation of synapsin I was monitored. Our results show that addition of bilirubin to the medium significantly decreases 32P incorporation into synapsin I, both under basal and depolarizing conditions, in a time- and dose-dependent manner, significant effects being observed already at 10 microM bilirubin after 120-min incubation of the synaptosomes. Separate analysis of the multiple phosphorylation sites in synapsin I showed that the phosphorylation of both the "head" and "tail" regions of the protein was decreased by bilirubin. Removal of the bilirubin-containing incubation medium retarded the decrease in synapsin I 32P content, indicating that the effect observed may be reversible. The nontoxic pyrrole biliverdin had no effect on synapsin I phosphorylation under the experimental conditions used, indicating that the effect was specific to bilirubin. Our results thus suggest that bilirubin may achieve some of its reversible effects on the brain through inhibition of the phosphorylation of the synapsic vesicle-associated protein synapsin I.