This paper tests the hypothesis that the local analgesic agent mepivacaine diffuses between adjacent equine synovial structures in the hindlimb and with greater frequency than latex, gelatine dye or contrast media. We report the incidence of diffusion of mepivacaine between the tarsometatarsal, centrodistal and tarsocrural joints, and the 3 synovial compartments of the stifle in 33 fresh equine cadavers. The tarsometatarsal joint and one synovial compartment of the stifle in the left limb and the centrodistal joint and a different synovial compartment of the stifle in the right limbs were injected with mepivacaine. Following flexion and extension of the limb, synovial fluid was aspirated from the noninjected centrodistal and tarsometatarsal joints and the tarsocrural joints of the hock and the noninjected compartments of the stifle. Concentrations of mepivacaine in these samples were assayed using an enzyme linked immunosorbent assay. For samples obtained by dilution of synovial fluid the concentration of mepivacaine was determined by comparing the concentration of urea in the diluted synovial fluid and the concentrations of the serum urea. Mepivacaine was detected in 25/25 (100%) adjacent tarsometatarsal and centrodistal joints after diffusion in both directions, in 23/25 (92%) of tarsocrural joints after diffusion from tarsometatarsal joints and in 22/25 (88%) tarsocrural joints after diffusion from centrodistal joints in the hocks. Diffusion from the femoropatellar to medial and lateral femorotibial joints and between the medial and lateral femorotibial joints in both directions were 20/20 (100%). Diffusion from the lateral femorotibial to the femoropatellar joint was 18/20 (90%) and from the medial femorotibial to femoropatellar joints 17/20 (85%). Mepivacaine was detected at concentrations >0.3 mg/l in a proportion of samples ranging from 15/25 (60%) in the tarsocrural joint following tarsometatarsal joint injection to 18/20 (90%) in the lateral femorotibial joint after femoropatellar joint injection. At mepivacaine concentrations >100 mg/l, detection ranged from 3/20 (15%) in the lateral femorotibial joint from the medial femorotibial joint to 19/25 (76%) in the centrodistal joint from the tarsometatarsal joint. At mepivacaine concentrations >300 mg/l, detection ranged from 1/25 (4%) in the tarsocrural joint from the tarsometatarsal joint to 16/25 (64%) in the from centrodistal joint the tarsometatarsal joint. The results show greater diffusion of mepivacaine between these adjacent synovial structures than assumed from previous anatomical, latex injection and contrast arthrographic studies. Therefore, commonly performed intrasynovial local analgesic techniques in the hindlimb of the horse are not as specific as first thought.