The course of induced tertian malaria has been studied in patients in psychoneurological hospitals in Moscow over a long period. Various strains of P. vivax were used to infect patients; the sporozoites were transmitted by Anopheles maculipennis atroparvus bred under laboratory conditions. There are marked differences between the dates of onset of relapses, reckoned from the primary manifestations of the disease, in patients developing malaria after long and short incubation periods. P. vivax strains which have a short incubation period are characterized by producing in patients a series of relapses following a long latent period; strains with a long incubation period are characterized by the occurrence of relapses during the first 3 months after the end of the primary series of attacks. However, some P. vivax strains give rise to both types of relapse, depending on the incubation period. In any event, the duration of the disease does not, as a rule, exceed 2 years, including the incubation period. The number of mosquitos (and hence the number of sporozoites) used to infect a patient does not have any noticeable effect on the number and frequency of relapses. In short-incubation tertian malaria, the use of quinocide during the period of treatment of the primary manifestations greatly reduced the number of relapses; in long-incubation tertian malaria similarly treated with quinocide, no relapses occurred.