Six metacarpo-phalangeal joints of adult horses were studied. Pressure measurements were made in the joint and the metacarpal bone with simultaneous measurement of the systemic arterial blood pressure. Investigations performed to study the effect of joint position on juxta-articular bone marrow pressure showed that an increase in joint flexion was always followed by a rise in intraosseous pressure with a significant increase at flexion above 60 degrees. Increase in intra-articular pressure which was achieved by injection of saline was always followed by a slower rise in intraosseous pressure. Furthermore, it was shown that even a few millilitres of saline in the joint caused a rise in intra-articular pressure. The findings indicate that changes in joint position as well as effusion may block the drainage vessels from the bone marrow as they pass through the joint.