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The botanical description and distribution of a Western Australian species of Swainsona is reported in connection with investigations of the plant as a poison plant responsible for extensive losses of sheep and horses in the Goldfields station country. Feeding trials in sheep and rabbits confirmed the toxicity of Swainsona. Sheep exhibited depression, anxiety, "blind staggers", incordination, and emaciation. The central nervous system of both animal species was seriously affected, with chromatolytic and vacuolar degeneration of neurones and with oedematous changes. Renal cells showed hydropic degeneration and spleens were intensely haemosiderotic. These changes occurred after 4 or 5 weeks of chronic feeding and closely resembled those of locoweed poisoning of livestock in the western United States.