Objective-To determine the effects of once-daily oral administration of N-acetyl-d-glucosamine (NAG) on plasma and urine glycosaminoglycan (GAG) concentrations in cats with idiopathic cystitis (IC). Animals-19 cats with IC and 10 clinically normal cats. Procedures-Cats with IC were randomly assigned to receive 250 mg of NAG in capsule form orally once daily for 28 days (n = 12) or a placebo (capsule containing cellulose) orally once daily for the same period (7). In cats with IC, plasma and urine GAG concentrations and urine creatinine concentration were measured on days 0 (immediately before first dose), 7, 14, 21, 28, and 56. For purposes of comparison, those variables were measured in 10 clinically normal cats on day 0. Results-Mean +/- SEM urine GAG-to-creatinine concentration ratios (day 0 data) for cats with IC and clinically normal cats differed significantly (3.11 +/- 0.62 μg/mL and 14.23 +/- 3.47 μg/mL, respectively). For cats with IC, mean plasma GAG concentration in NAG-treated cats (39.96 +/- 5.34 micrograms/mL) was higher than that in placebo-treated cats (24.20 +/- 3.35 micrograms/mL) on day 21. In the NAG-treated cats, plasma GAG concentration on days 21 (39.96 +/- 5.34 micrograms/mL) and 28 (39.91 +/- 6.74 micrograms/mL) differed significantly from the day 0 concentration (27.46 +/- 3.90 micrograms/mL). Conclusions and Clinical Relevance-Cats with IC have lower urinary GAG-to-creatinine concentration ratios than did clinically normal cats. Administration of NAG (250 mg, PO, q 24 h) significantly increased plasma GAG concentrations in cats with IC after 21 days of treatment.