The objective of the present study was to identify a consistent method for measuring the equine metacarpophalangeal joint (MCPJ) angle in a standing horse; and investigate effects of lifting the contralateral limb, changing head position and treadmill exercise on that angle. A circle circumference was best fit onto lateromedial radiographs of the dorsal region of the lateral condyle of three MCPJs with radio-opaque markers fixed on the bony prominences. The circle center was assumed to approximate the center of MCPJ motion and the bony eminence at this point was used to position the goniometer in subsequent tests. MCPJ dorsal angles of ten vertically-fixed cadaver forelimbs were measured with a goniometer five times by rater A, and once by rater B. MCPJ angles of fourteen horses were measured before and after lifting the contralateral limb. Five (standing) and eight (limb lifted) horses were measured six and five times in random order respectively. Skin marked MCPJ of five horses were filmed in five different head positions (forward, right, left, raised, and lowered). Duplicate MCPJ angles of 6 control and 6 exercised horses were measured before and after treadmill exercise. The goniometer repeatability coefficient was 4.7° (95%CI 3.9° to 6.0°) and was similar in horses standing squarely and with the contralateral limb lifted. There was no significant difference in MCPJ measurements between raters. MCPJ dorsal angles were significantly less with the contralateral limb lifted. There was no significant effect of head position. The right MCPJ dorsal angle decreased with exercise. In conclusion, the goniometer can be used to measure MCPJ angles in live horses, either standing squarely or with the contralateral limb lifted. Recent exercise, but probably not head position may affect the angle.