Abstract Thirty ewe lambs were divided into three equal groups (A, B and C) and inseminated after synchronization of their oestrous cycles. Their sera were tested for antibodies to Chlamydia psittaci (ovis) at fortnightly intervals by a complement fixation test. Group A was challenged with C. psittaci (ovis) by the oral route two months after insemination. Group B was challenged by mixing chlamydiae with semen immediately prior to insemination. Group C was isolated from infection until the end of the first pregnancy when all groups were mixed and housed intensively. The lambing environment was contaminated with chlamydiae excreted by two ewes from group A which showed clinical signs of ovine enzootic abortion (OEA) at parturition. Ewes that had not conceived were separated after the lambing period and challenged again by simulated venereal transmission while they were in oestrus. They were slaughtered four months later. No chlamydiae could be demonstrated in the genital tract. The ewes that had lambed were isolated from further challenge and were mated. At the end of their subsequent pregnancies two ewes from group C aborted with clinical signs of OEA. Eight ewe lambs (group D) born in the contaminated perinatal period were isolated after weaning and were mated. One of them produced a stillborn lamb with clinical signs of OEA. Most sheep seroconverted after initial challenge with chlamydiae but their titres fell rapidly. Seroconversion was most obvious in all groups after challenge in the contaminated perinatal period. The titres of those sheep that displayed OEA rose at the time of clinical disease and remained high. The results demonstrate that infection contracted in the perinatal period manifests itself in the next pregnancy of some ewes. They indicate that lambs may become infected before they are mated, probably in their own perinatal period.