Urinary excretion of N-acetyl-beta-D-glucosaminidase (NAG) was examined in healthy dogs and dogs with urinary diseases, and its clinical usefulness as an indicator of urinary diseases was discussed. Twenty-eight healthy dogs and 20 dogs with urinary diseases were used. Urinary NAG activity was measured using p-nitrophenyl N-acetyl-beta-D-glucosaminide as substrate, and expressed as units per gram of urinary creatinine (NAG index). Urinary NAG index in urine of healthy dogs was 3.2+/-2.4 U/g, and NAG index in the dogs with chronic renal failure or lower urinary tract infection accompanied by pyelonephritis was higher than that in healthy dogs. However, the dogs with lower urinary tract infection without pyelonephritis showed normal values of NAG index. Some dogs with diabetic mellitus showed elevated values of NAG index when control of blood sugar was not successful. Increase of NAG index was observed in some dogs with pyometra before increases of BUN and serum creatinine concentration. Therefore, NAG index in urine seems to be a good indicator for urinary diseases in dogs.