Summary and Conclusions Renewed clinical activity after the spontaneous or induced termination of the primary attack has been observed to occur with about ten times greater frequency in patients inoculated with the McCoy strain of P. vivax than in those inoculated with other strains. Renewed clinical activity thus appears to be a characteristic which varies markedly with different strains of this parasite. Patients having primary attacks of more than 48 days’ duration have not experienced renewed activity, and renewed activity is only rarely observed in persons whose primary attacks last for less than seven days. Renewed activity is most often associated with primary attacks of from 7 to 34 days' duration. Renewed clinical activity is most often observed within 8 weeks of the termination of the primary attack. The period of renewed activity is usually of shorter duration than the primary attack, but it may sometimes be longer. The first such period is usually longer than the second or later periods. Patients experiencing renewed activity have incubation periods of duration similar to the series as a whole. An interval of quiescence of 5 days, after a previously unbroken series of quotidian or tertian paroxysms since the onset, is not a suitable criterion of the termination of the primary attack. The termination of the primary attack is probably marked by an interval of quiescence of at least 2, or perhaps 3, weeks. Periods of renewed activity occurring within approximately 60 days of the onset are essentially a part of the primary attack. Renewed activity was observed in slightly more than 50 per cent of the patients whose primary attack terminated spontaneously and who did not subsequently receive intensive therapy. When termination of the attack was followed by intensive therapy, cases of this character have not shown further activity. Patients whose primary attacks were interrupted by small doses of quinine have invariably had a return of clinical activity unless they subsequently received intensive therapy. Renewed activity after a quiescent interval of from 8 to 24 weeks following the primary attack has been noted chiefly in the fall in patients who were inoculated in the spring. Renewed activity after 24 weeks of quiescence has been noted chiefly in patients who were inoculated in the fall and occurs in the following spring. Epidemiologically this renewed activity appears to be an important means of carrying the species over periods when conditions are unfavorable to transmission.