BACKGROUND: Urine sodium concentration (uNa) measures diuretic responsiveness (DR) in acute congestive heart failure (CHF). In humans with chronic CHF receiving loop diuretics PO, low uNa is associated with rehospitalization and mortality. DR in dogs with chronic CHF is poorly understood. HYPOTHESIS/OBJECTIVES: Measure uNa and pharmacologic and pathophysiologic variables in dogs receiving loop diuretics PO for chronic CHF. ANIMALS: Eleven client-owned dogs. METHODS: Prospective longitudinal observational study. Owners collected multiple convenience urine samples over 5 days. Associations between urine electrolyte, creatinine, and loop diuretic concentrations, dose and duration of PO diuretic treatment, and time from previous diuretic administration or feeding were examined using mixed effects linear regression. RESULTS: A total of 189 samples were collected. Median uNa was 27 mmol/L (IQR, 10-61 mmol/L). Dose and time from previous PO diuretic or feeding were not significantly associated with uNa. In 5 of 11 (45%) dogs, uNa < 70 mmol/L was observed consistently and indicative of low DR. Urine diuretic concentration was not significantly associated with uNa (β, 0.29; 95% CI, -0.03 to 0.61; P = .08) or diuretic dose, duration of treatment, or time from last PO diuretic or feeding. Individual variability was high. Observed patterns of uNa, diuretic dose, and urine diuretic concentration suggested potential causes of low DR. CONCLUSIONS AND CLINICAL IMPORTANCE: Measurement of uNa and diuretic concentration identifies dogs with low natriuresis, which is not predicted by diuretic dose or duration. Observation of uNa patterns generates hypotheses regarding potential causes of low DR that warrant confirmation in larger studies.