Syncope with and without convulsion was studied in unselected blood donors in a community blood center. Convulsive syncope occurred in 0.03% of all blood donors and was more commonly observed when nursing personnel were alerted to its possible occurrence. It was more common in men. Although tonic extensor spasm was the most common convwithout convulsion was studied in unselected blood donors in a community blood center. Convulsive syncope occurred in 0.03% of all blood donors and was more commonly observed when nursing personnel were alerted to its possible occurrence. It was more common in men. Although tonic extensor spasm was the most common convwithout convulsion was studied in unselected blood donors in a community blood center. Convulsive syncope occurred in 0.03% of all blood donors and was more commonly observed when nursing personnel were alerted to its possible occurrence. It was more common in men. Although tonic extensor spasm was the most common convulsive movement, other complex convulsive phenomena occurred, some simulating epileptic seizure. No statistical difference in changes of pulse or blood pressure was found between subjects with convulsive versus nonconvulsive syncope. Similarly, no difference was found between subjects with tonic spasm and those with other convulsive phenomena, nor between those with "early" and those with "delayed" reactions. Marked individual variation may exist in the susceptibility of the central nervous system to ischemia. Some individuals appear to be predisposed to development of seizures in situations of global cerebral ischemia such as occur in hypotension and bradycardia.