The transmission of maedi/visna from a severely infected flock to their progeny was studied. At birth, the lambs were divided at random in four groups of approximately forty animals each, which were exposed to the parent flock for zero hours, ten hours, six weeks and one year respectively. Although more than 80 percent of the ewes were affected with maedi, lambs which were separated from the ewes immediately after birth continued to be free from maedi/visna virus infection during an eight years period of observation. In addition, antibodies to the virus were not detected in this group. It is therefore concluded that congenital infection, if any, is of minor importance in the epizootiology of the disease. The number of sheep in which serological, virological and histopathological studies were positive, the onset of the disease and the severity of the lesions in the three other flocks increased with the period for which they had been exposed to the ewes. A separate trial did not supply any evidence to suggest that larvae of Muellerius capillaris are capable of transmitting infection with maedi/visna virus.