Sera from acute and chronic cases of natural grass sickness or normal horses were injected into the parotid salivary gland of ponies. This gland receives its sympathetic innervation from the ipsilateral cranial cervical ganglion. None of the ponies showed any local or systemic signs of illness. After one week the cranial cervical ganglia, stellate and coeliaco-mesenteric ganglia were removed for histological study. Pathological changes were found only in the cranial cervical ganglion ipsilateral to a parotid salivary gland which had received an injection of grass sickness serum. Four out of five batches of test sera from cases of acute natural grass sickness were associated with chromatolytic changes in neurons; the remaining batch of serum produced no abnormalities. The most severe chromatolytic changes were induced by two samples obtained from horses whose signs of grass sickness had been present for less than 12 hours. A serum sample from a chronic case of grass sickness of three weeks duration did not produce chromatolysis but was associated with a moderately severe inflammatory infiltrate and neuronophagia in the ipsilateral cranial cervical ganglion. One batch of serum was size fractionated to separate components with molecular weights above or below 30 kDa. Only the fraction containing components above 30 kDa induced chromatolytic changes.