This article describes a case of massive mortality among horses which was probably due to intoxication by pyrrolizidine alkaloids fromHeliotropium indicum. Over 4 years more than 75% of a population of about 110 horses on a farm in Costa Rica died after showing nervous neurological symptoms. Two clinical manifestations were encountered, an acute and a chronic one, both with a fatal outcome. Pathological findings in 2 horses coincided with those reported in the literature for intoxication by pyrrolizidine alkaloids and were not specific for VEE. However Venezuelan equine encephalitis (VEE) was the main differential diagnosis and could not completely be excluded because this disease was endemic in the region and VEE titres were found to be high. Taxonomic and toxicological investigations implicatedHeliotropium indicumas the most probable principal cause of the intoxication.