We measured regional blood flow in synovial tissue of the antebrachiocarpal, midcarpal, and metacarpophalangeal joints of six normal adult anesthetized horses by using 15-mu m-diameter polystyrene colored microspheres. The midcarpal fibrous capsule and synovial membrane blood flows (SMBF) were compared, and the effect of increased intra-articular pressure (30 and 60 mmHg) on midcarpal SMBF was investigated. Dorsal, medial palmar, and lateral palmar midcarpal SMBF measured 108 +/- 36, 61 +/- 12, and 50 +/- 11 mu l . min(-1). g(-1), respectively. Antebrachiocarpal, dorsal, and palmar metacarpophalangeal SMBF measured 103 +/- 8, 17 +/- 3, and 26 +/- 5 mu l . min(-1). g(-1), respectively. Midcarpal fibrous joint capsule blood flow was significantly lower than that of the synovial membrane. An increase in midcarpal intra-articular pressure to 30 or 60 mmHg resulted in an 84% decrease in SMBF. Colored microspheres provided a useful technique to determine sequential SMBF. Increased intra-articular pressure significantly altered SMBF, suggesting a role of the regional circulation in the pathogenesis of joint disease.