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To (1) investigate the tissue response to a novel urethral bulking agent, polyethylene glycol carboxymethyl cellulose hydrogel (PEG-CMC) injected submucosally in the canine urethra and (2) compare PEG-CMC with bovine collagen (BC), the current standard for urethral bulking.Experimental study.Purpose-bred female hound dogs (n = 8).Standardized submucosal urethral injections of BC and PEG-CMC were performed in 8 female dogs. Injection sites were evaluated by cystoscopy on days 0 (n = 8), 30 (n = 4), and 90 (n = 4), magnetic resonance imaging on days 0 (n = 8), 30 (n = 8), and 90 (n = 4) and by histopathology on days 30 (n = 4) and 90 (n = 4).Both PEG-CMC and BC were detectable on MRI as hyperintense foci on T2-weighted images. Grossly, PEG-CMC formed more prominent blebs than BC. On follow-up cystoscopic examination, 6/8 PEG-CMC injection needle tracts were visible, and 3 of these sites had mucosal erosions. Histopathologic scores for foreign body reaction and inflammation were significantly higher for PEG-CMC compared with BC (P < 0.005). BC elicited a lymphoplasmacytic reaction whereas PEG-CMC incited a granulomatous response.The overall physical characteristics and histologic response associated with PEG-CMC support its use as a urethral bulking agent; however, the current formulation needs to be adjusted for clinical use.