The effect of nerve growth factor (NGF) on tunicamycin (Tm)-treated neurons in the trigeminal ganglion was investigated by use of caspase-3 immunohistochemistry. In intact embryos at embryonic day 16.5, only a few caspase-3-immunoreactivity were detected in the ganglion neurons. Mean +/- A SE of the density of the immunoreactivity was 0.22 +/- A 0.03%. In contrast, the number of the immunoreactive neurons was increased at 24 h after injection of 0.5 mu g Tm in 1 mu l of 0.05 N NaOH solution into mouse embryos at embryonic day 15.5. The density of immunoreactivity was also increased (mean +/- A SE = 1.44 +/- A 0.11%) compared to intact and 0.05 N NaOH-treated embryos (mean +/- A SE = 0.35 +/- A 0.03%). The Tm treatment caused increase of the number of trigeminal neurons representing apoptotic profiles (intact, mean +/- A SE = 79.3 +/- A 8.5; 0.05 N NaOH, mean +/- A SE = 132 +/- A 11.5; 0.5 mu g Tm, mean +/- A SE = 370.2 +/- A 64.8). In addition, NGF significantly prevented the increase of density of the immunoreactivity (mean +/- A SE = 0.54 +/- A 0.16%) and the number of apoptotic cells (mean +/- A SE = 146.2 +/- A 11.3). Saline application (without NGF) had no effect on Tm-induced increase of the immunoreactivity (mean +/- A SE = 1.78 +/- A 0.23%) or the apoptotic profiles (mean +/- A SE = 431.9 +/- A 80.5). These results indicate that Tm-induced cell death in the trigeminal ganglion is suppressed by NGF in the mouse embryo.