The apoplosls occurring in the postnatal developing spiral ganglion cells (SGCs) and Ihe concomitant change in Bax-prolein immunmeuctlvity were investigated in rats at postnatal days (P) I, 3, 5, 7, l0 and 14, and also in adull rats In P5 and P7 rals, some neurons showed structural features of poptosis, includlng cell shrinkage, condensed chromatin around the margin of the nucleus and phagocytosis of apoptotic bodies by satellite coils. The percentage of Bax-protein- positive neurons ill the spiral ganglion was significantly higher in PI, P3, PS, lind P7 rat5 compared with adult rats Our resulls conflm the oecurrace of neuronad cell apoptosis in the splmal ganglion at the end of the first ostllatal week, and demonstrate the trallsJent chalge in Bax-plotein immunorcactlvity prior Io neuronal cell apoplosJs Thus, we speculate that the Iransient change in Bax protein could be involved in neuronal cell apoptosis of the postnatal developing SGCs of rats.