‘Mal seco’ is an almost invariably fatal disease of horses in Argentina and Chile, which resembles grass sickness, a dysautonomia of horses in Europe. The aetiology of mal seco remains unknown. An attempt to reproduce the disease was made by feeding horses with Festuca argentina, a plant considered to be toxic to animals and which was consistently found in the diet of nine horses suffering from mal seco. Three horses were fed with F argentina ad libitum for 28 days. The plant was infected with an endophytic fungus, whose morphological characteristics were in agreement with descriptions of Acremonium chlamydosporioides. No clinical abnormalities were observed in two of the horses, but one died on the fifth day of the trial after becoming incoordinated, unsteady and ataxic in the fore- and hindlimbs. No gross changes were observed post mortem in any of the horses, with the exception of a small number of Fasciola hepatica in the liver of the horse which died, and a moderate number of Gasterophilus species in the stomach of all three horses. No histopathological changes were observed in any of the organs examined, including several autonomic ganglia, brain including most brain stem nuclei, spinal cord, liver, kidney, stomach and small and large intestine. The results of this study suggest that F argentina is either not implicated in the aetiology of mal seco or produces its effects only when they are triggered by other unknown factors.