During programmed cell death (PCD) apoptosis is controlled by many factors such as proteases. With no specific protease (s) known during PCD in the developing nervous system so far, we sought to determine if any specific protease (s) is involved in this process and therefore used different protease inhibitors during PCD (from embryonic day 6 to 10) in chick embryos. Among the inhibitors commercially available, those for the inhibition of caspase 8, caspase 9 and calpain were used from embryonic day 6 to 9. Embryos were injected daily with 10 ?g of the inhibitors, killed at embryonic day 10 with their ciliary ganglia (CG) excised and neurons counted. The results in this study show that only inhibition of caspase 9 but not that of caspase 8 or calpain results in a significant increase in neuron numbers of the ciliary ganglia. These results suggest a key role for caspase 9 in the intracellular pathway of programmed cell death.