To study the influence of variations in mean arterial pressures (MAP) on long bone medullary pressures, seven anaesthetized dogs were investigated. The medullary pressures were measured in the epiphyseal, the metaphyseal and the diaphyseal regions and remained rather constant when MAP was above 80 mmHg. Below this level of MAP a statistically significant (P less than 0.01) reduction of the medullary pressures was seen. Comparing the mean medullary pressures obtained with ranges of MAP of 81--100 mmHg (the control medullary pressures) and of 61--80 mmHg, the greatest decline was seen in the epiphyseal and the diaphyseal regions, from 25.2 mmHg to 8.1 mmHg and from 26.7 mmHg to 8.3 mmHg, respectively. The corresponding decrease in the metaphyseal region was from 18.9 mmHg to 10.9 mmHg. The mean values of intraosseous pressure measured by our technique were between 20--30 mmHg and this is in accordance with measurements in normal humans found by other authors.