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Background: High‐salt diets promote urine dilution and decrease urolithiasis risk. Objective: Prospectively evaluate the safety of chronic high dietary salt intake (randomized controlled trial). Animals: Twenty research colony neutered, healthy aged cats (11.5 years [10.0‐11.6], median [interquartile range]). Methods: Healthy cats were randomized to control or high‐salt dry diets (sodium: 1.02 ± 0.16 [mean, SD] and 3.26 ± 0.30 g/Mcal metabolizable energy [ME], respectively; chloride: 2.26 ± 0.33 and 5.71 ± 0.28 g/Mcal ME, respectively), fed for up to 60 months. Assessments included CBC, plasma biochemistry, urinalysis, glomerular filtration rate (GFR), blood pressure, renal and cardiac (conventional Doppler and 2‐dimensional color tissue Doppler) imaging, annually. Cats that died or were euthanized underwent necropsy. Diet effects over time were evaluated with linear mixed models. Results: Follow‐up duration (median [Interquartile range]) was similar between the control (38.7 months [28.6‐48.2]) and high‐salt group (51.4 months [45.7‐59.0]). Diet had no significant effect on changes in GFR, blood pressure, plasma creatinine concentration, end‐diastolic left ventricular (LV) wall thicknesses, LV internal diameters, LV systolic function, left atrial size, or systolic and diastolic Doppler variables. One control cat developed hypertension. One high‐salt group cat developed persistent azotemia. Serial plasma biochemistry and urine specific gravity suggested early chronic kidney disease in 4 nonazotemic cats (2 per group), consistent with necropsy findings. Conclusions and Clinical Importance: In healthy aged cats, a commercial veterinary diet containing 3.26 ± 0.30 g/Mcal ME sodium was safe with regard to renal and cardiac function for up to 5 years. [ABSTRACT FROM AUTHOR]