A strain of the virus of poliomyelitis has been described which has passed through several stages of virulence as tested upon monkeys. The first stage consisted of the adaptation of the original human virus to the monkey. In this process high virulence was readily achieved. The adapted, virulent strain of virus was passed regularly through monkeys and maintained its activity for about 3 years, when diminution became apparent. The loss of power of the virus was such that it may be said to have returned approximately to the level of the original human virus. This change constituted the second stage. The third stage is represented by recovery of the high virulence. This revival occurred, it seems, during the sojourn of the virus in glycerol and required several years for its consummation. It was first noticed nearly 6 years after the low level of the second stage became established. The potent virus of the third stage has been found to remain active over a period of at least 4 years while preserved in glycerol. What constitutes at least a superficial resemblance between the wave-like rises and falls of the incidence of epidemic poliomyelitis and the phenomena of increase and decrease in virulence of the specimen of virus has been alluded to. The two processes differ, however, essentially in respect to the time factor, since the fluctuations of the epidemic wave occupy small and those of the virulence occupy large increments of time.