Hyperbaric oxygen toxicity studies were conducted on rabbits using the opsonic index determination.The study was conducted on 15 animals that had opsonin index examined prior to hyperbaric oxygen exposure. They were then subjected to an hourly exposure to hyperbaric oxygen with overpressure values of 1.8, 2.4 and 3.1 atm in groups of 5 animals. After the exposure, the opsonium index was re-examined upon the lapse of 1, 2 and 10 days. Parallelly, the morphological image of the blood was examined.There was a statistically significant increase in the index in the first two days after exposure, independent of the value of oxygen overpressure. On the 10th day, the index value approached the initial one.