Following intranasal inoculation of three groups of pregnant swine (in total 11 dams) with a Danish isolate of porcine reproductive and respiratory syndrome virus (PRRSV) on or about day 85, 70 and 45 of gestation, respectively, reproductive disturbances were observed in the first two groups. Transplacental transmission of PRRSV occurred in four out of five litters from dams inoculated around day 85 of gestation and in two out of three litters from dams inoculated on day 72 of gestation. In the third group, inoculated around day 45 of gestation, transplacental infection could not be demonstrated. Thirtytwo (56%) piglets from dams inoculated on day 85 of gestation and 14 (33%) piglets from dams inoculated on day 72 of gestation, were transplacentally infected. Sixteen (28%) and six (14%) piglets, respectively, in these groups became infected in the perinatal period. Thirtytwo (56%) piglets from dams inoculated on day 85 of gestation were stillborn or died within a 6-8 weeks observation period, 29 being stillborn or dying within the first two weeks of observation. Thirteen (30%) piglets from dams inoculated on day 72 of gestation died within the two weeks observation period. The duration of the viraemic phase varied considerably, from one day to four weeks, for both darns and their offspring. Most frequently, PRRSV was isolated from lung and/or tonsil tissues from dead and euthanized piglets younger than 14 days of age. Histopathological investigations of piglets typically revealed focal nonsuppurative inflammatory conditions, especially in the lung and heart. In conclusion, the present results support the hypothesis, that PRRSV infection of dams late in pregnancy has the greatest likelihood of transplacental infection of fetuses.