A field population of English sparrows (Passer domesticus) was used to test the hypothesis that the reported spring maximum in the prevalence of Plasmodium infections is the result of an elevation of circulating parasite populations in established infections. Sparrows were captured, banded, infected with Plasmodium relictum, and released. Since natural infections were negligible in this population, patent infections in recaptured birds were assumed to result from the inoculation of parasites. The primary parasitemia in the experimental birds lasted approximately 25 days, while a substantial proportion of low intensity infections persisted through 125 days. Analysis of the seasonal distribution of patent infections in birds infected 125 days or longer showed a sharp rise in both prevalence and parasitemia in the spring. This study thus supports the hypothesis that spring relapse of established infections is responsible for the high spring prevalence of Plasmodium in birds.