The effect of initial Haemoproteus belopolskyi infection on the weight of its natural host, the blackcap Sylvia atricapilla, was investigated. Fourteen blackcap nestlings were taken at the age of 4–5 days and raised by hand in the laboratory. They were free of blood parasites. Seven 20- to 21-day-old nestlings were infected experimentally by inoculation in their pectoral muscle with approximately 45 sporozoites, which had developed in the experimentally infected biting midge Culicoides impunctatus. Seven nestlings were used as negative controls. Parasitemia developed in 6 inoculated nestlings, with a prepatent period of 11–12 days. No infections were detected in the controls during this study. The weight of experimentally infected and control birds was measured 2 days before parasitemia became patent and for a 45-day period after patency. Blood smears were prepared from all birds on the days when they were weighed. When compared with controls, there was a significant weight loss of experimentally infected blackcaps during 6 days after the decline of parasitemia at 10–16 days of patency, indicating a short-term influence of the infection on the birds' body mass. Clinical symptoms of the infection were not recorded. All birds from both groups survived until the end of the experiment.