22 juvenile rhesus macaques were infected i.v. with SIVmac and killed at defined timepoints after infection. Productively infected cells were detected by RNA in situ hybridization in the paraffin material. Their number was correlated with the pathology of lymph nodes, thymus, extranodal lymphatic parenchyma and other organs. In the first weeks all lymphatic tissues and compartments got infected, as well as the brain, the bone marrow and other organs. The high virus replication during this first phase disappeared with the onset of the seroconversion and remained low during all stages of atrophy of the lymphatic parenchyma. The atrophy of the lymphatic parenchyma and its microenvironment was not correlated with virus replication. This may implicate that a virostatic therapy might be more successful in the first weeks of infection.