Summary. Proviral DNA load and expression of avian leukosis viruses of subgroup C (ALV-C) in ducks infected in mid embryogenesis were studied using quantitative PCR, RT-PCR, in situ hybridization employing ALV-specific riboprobe, and immunohistochemistry. A group of long-term surviving, non-reviremic ducks was selected for the study and compared to control reviremic animals in order to obtain information about persisting retroviruses in different duck tissues. A widespread distribution of proviruses in the tested tissues was found, but the proviral load was significantly lower in non-reviremic in comparison to reviremic animals. The only exception were brain and blood cells, in which no significant difference in the quantity of integrated proviruses was found between both categories of ducks, thus indicating an exceptional position of the brain and blood cells among all tested tissues. Contrary to reviremic, the proviruses were not transcribed in non-reviremic ducks, with the exception of brain and thymus. In the majority of non-reviremic ducks viral RNA was revealed in the brain, but no infectious virus could be recovered from this tissue. The opposite situation was observed in the thymus, where infectious virus was recovered but viral RNA remained below the detection limit of the assay. As revealed by in situ analysis, infected cells were either disseminated or focally distributed in tissues. From the long-term follow up of ALV-C in intraembryonally infected ducks we conclude that this model is suitable for the study of retrovirus persistence accompanied both by the presence and absence of reviremias. The possible consequences of transmission and long-term persistence of retroviruses in the heterologous host for retroviral evolution are discussed.