High-speed microcinematography of nematocyst discharge in Hydra attenuata Pall. shows that this specialized exocytosis, which appears to result from an increase in the intracapsular pressure, requires a total of about 3 milliseconds. A maximum velocity of 2 meters per second is generated, corresponding to an acceleration of 40,000g. Thus nematocyst discharge is one of the fastest cellular processes in nature.