A 2.5-cm long, 0.8 mm in diameter catheter was placed percutaneously into the palmarolateral pouch of the antebrachiocarpal joint in 6 clinically normal horses. The catheter was affixed in place for 72 hours. Cytologic analysis was performed on synovial fluid specimens obtained through the catheter at postcatheterization hours (PCH) 0, 24, and 72. The horses were euthanatized at PCH 72, and macroscopic and microscopic examinations were performed on the dorsal portion of the joint capsule and the palmarolateral pouch of the catheterized and contralateral (noncatheterized) joint. Clinical, synovial fluid cytologic, and synovial membrane histologic examinations were performed to assess the effect of the catheter on clinically normal equine synovial membrane. Serially obtained synovial fluid specimens were yellow and clear or hazy and had good mucinous precipitate quality at all times in all horses, except 2, in which the catheter required readjustment. Mean refractive index was slightly decreased, and the RBC count was high at PCH 24 and 72, compared with PCH 0; the highest RBC count was 12,550 cells/microliter (PCH 24). Statistically significant (P less than 0.05) increases were observed in WBC, neutrophil, and large and small mononuclear cell counts between PCH 0 and 72. These increases were modest, except the mean WBC count (51,000 cells/microliter, PCH 72) observed in 1 horse in which the catheter was dislodged, requiring reinsertion into the joint. At necropsy, subcutaneous hemorrhages were observed at the catheter insertion site in all horses. The synovial membrane of the catheterized joint was discolored (ranging from yellow-orange to salmon), compared with the contralateral synovium (noncatheterized joint).(ABSTRACT TRUNCATED AT 250 WORDS)