The results of this investigation reveal, for the first time, the presence of thymus-derived (T) lymphocytes in the trigeminal ganglia of rabbits undergoing primary corneal infection with herpes simplex virus type 1. Infiltration of T cells into the trigeminal ganglion was evident at 15 days after primary ocular infection but these cells were no longer present by 45 days after infection. Corneas and trigeminal ganglia of rabbits sacrificed at 3, 7, 12, 15, 20, 25, 30, 45, 70 and 90 days after infection were assayed for infectious virus and stained for viral antigen and immunoreactive T cells. Infectious virus and cells expressing viral antigens were present in the corneas and trigeminal ganglia during the acute phase (day 0-day 14) of the infection. T cell infiltration of the trigeminal ganglion was present as a perivascular infiltrate along with a sparse scattering of these cells among the nerve fibers. The perivascular infiltration is characteristic of viral infection of a tissue and was not seen in the sections of trigeminal ganglia obtained earlier than 15 days or in ganglia obtained 45 days or more after primary corneal infection. This investigation demonstrates conclusively that the neural ganglia are not completely shielded from the host immune response, as evidenced by the observation that immunocompetent T lymphocytes infiltrate the ganglia subsequent to the infection of a peripheral tissue such as the cornea of the eye.