This article reports an outbreak of intoxication of female horses with Claviceps purpurea in southern Brazil. The outbreak affected twelve pregnant mares which were fed with black oat (Avena strigosa) during the pre-delivery period. Underdevelopment of the mammary gland in the pre-delivery period resulting in post-delivery agalactia was the most pronounced finding. These mares delivered weak and unviable foals, which showed no suckling reflex and died within a few hours of birth. Laboratory analysis of oat samples fed to the animals resulted in the identification of Claviceps purpurea sclerotia. The fungus was identified in 0.22% of the examined seeds.