found this:
Conditions of poisoning All parts of the plant, but especially the foliage, contain the poison, and two or three leaves may produce severe toxicosis. Sucking flowers free of nectar may produce serious illness. Rhododendrons are more likely to retain green leaves year round than are most other plants, and therefore most toxicoses occur in the winter and early spring, when other forage is unavailable.
Toxic principle All parts of this plant contain toxic resins (andromedotoxins, now commonly referred to as grayanotoxin) with the leaves being the most potent. Grayanotoxin produces gastrointestinal irritation with some hemorrhage, secondary aspiration pneumonia, and sometimes renal tubular damage and mild liver degeneration.
Clinical signs Clinical signs usually appear within 6 hours of ingestion. Affected animals may experience anorexia, depression, acute digestive upset, hypersalivation, nasal discharge, epiphora, projectile vomiting, frequent defecation, and repeated attempts to swallow. There also may be weakness, incoordination, paralysis of the limbs, stupor, and depression. Aspiration of vomitus is common in ruminants and results in dyspnea and often death. Pupillary reflexes may be absent. Coma precedes death. Animals may remain sick for more than 2 days and gradually recover.