The time course of inhibition of potato virus X (PVX) synthesis by the newly developed antiphytoviral compound 2,4-dioxohexahydro-1,3,5-triazine (DHT) was determined in mechanically inoculated leaves of Nicotiana tabacum L. cv. ‘Samsun’. At the permissive temperature (22 ± 3°) DHT inhibited the synthesis of PVX almost 100% within 8 h postinoculation, after which the inhibition declined. In leaves maintained at a temperature of 5 ± 2° for 5 days, DHT inhibited PVX synthesis about 65% immediately after the shift to permissive temperature. This time course of inhibition was almost identical to that of 2-thiouracil. Uracil completely overcame the inhibition of PVX replication caused by DHT when added within 2 h after incubation of leaf disks with DHT. These results indicate that DHT acts as an analogue of the pyrimidine base uracil.