This study was Uesigned to evaluate the reflexive heart rate response to acute blood loss during sodium nitroprusside and prostaglandin E1 induced-hypotension in isoflurane anesthetized dogs. The results were as follows: 1) Reflexive increase in heart rate during induced hypotension was significantly greater in sodium nitroprusside than prostaglandin E1. 2) Reflexive increase in heart rate during induced hypotension was significantly greater prostaglandin E1 induced-hypotension than during that wih sodium nitroprusside. From these results, it suggest that prostaglandin E1 induced-hypotension provides a safer margin than with sodium nitroprusside when rapid bleeding occurs during anesthesia and surgery.