The course of infection and IBR virus reactivation was studied in three experimentally infected weaned calves and three cows from naturally invaded herds. The animals were infected intranasally, intratracheally, and by contact. After 20, 41, and 105 days from primary infection, both in calves and in cows, dexametazon was applied in a series of six to seven intramuscular injections. The presence of the virus was examined in the nasal, conjunctival, vaginal and/or preputial secretions and in blood on diploid cells of calf kidneys and by the immunofluorescence method. In all infected calves, the disease took place with clinical signs of rhinotracheitis, mostly within the period of nine days. The second and third day after a temperature rise, the virus titre in nasal secretion reached the values ranging from 10(5) to 10(6.5). A markedly lower titre was obtained in the conjunctival secretion 10(0.5) to 10(3.5). In blood, the virus was found to be present on the first and fifth day from infection. After dexametazon application the calves and cows eliminated the virus mainly with the nasal secretion whose titre highly rose to the value of 10(3.5) to 10(47). In the conjunctival secretion the virus was present only irregularly and its quantities were very small. The greatest quantities of the virus were found in the nasal secretion on the sixth to the eight day from dexametazon application. The virus was not found in vaginal and preputial secretions. The levels of neutralizing antibodies were not affected by dexametazon in the calves; in cows they rose significantly from the titres of 1:2--1:4 to the titres of 1:16--1:32.