A female F344 rat was injected with 2.4 x 10(6) MT-2 cells intravenously at 3 and 4 weeks old, and was mated with a non-infected male rat at the 17th week after injection, when the dam rat contained HTLV-1 provirus in the peripheral blood mononuclear cells as determined by polymerase chain reaction. HTLV-1 provirus was detected in at least 2 out of 9 offspring. This system should be useful for studies on the routes and prevention of vertical transmission and on elimination of once-transmitted HTLV-1.