Six cases of paresis occurred in a Swedish stud with 48 mares and a stallion. Complement-fixation tests revealed a recent infection with EHV-1 in most horses of the stud. Serumneutralisation tests showed rapid antibody-titre increases during the course of the disease. This type of antibody response was interpreted as induced by reinfection or, possibly, recurrent infection. Two diseased mares were sacrificed. No virus could be isolated from their central nervous system (CNS), liver or spleen, but there is a presumptive evidence for the presence of an antigen specific to EHV-1 in the CNS and liver. Neutralising antibodies to EHV-1 were demonstrated in the liver and kidneys following elution by acidification of the tissues. No such antibodies could be demonstrated in the brain and spinal cord. A possible reason for this failure is discussed.