Daily observations were made on the excretion of African swine fever (ASF) virus by pigs infected intranasally or by contact. Two strains of virus having mean death times of approximately 3 and 6 days were used, the latter being recently isolated from a warthog.First excretion usually occurred by the nasopharyngeal route, as early as 1 or 2 days before the onset of fever in many cases. The titres of pharyngeal and nasal swabs rose rapidly to reach mean levels of about 10(4)-10(5) HAD 50 at 48-72 hr. following the onset of pyrexia. Virus in the secretions of the conjunctiva or lower urogenital tract appeared later and did not attain such high levels. Faecal and urinary excretion was of relatively little significance, except in slower infections caused by the recent warthog virus.These results are discussed in relation to the known failure of infected pigs to transmit the disease to stallmates during the first 12-24 hr. of pyrexia and also in relation to recent work on the pathogenesis of ASF in domestic swine.