Purpose The aim of this study was to define if tadalafil causes detrusor muscle impairment and to observe the effect of combination of tadalafil with tamsulosin on the lower urinary tract of rats with bladder outlet obstruction (BOO) induced by chronic nitric oxide deficiency. Materials and Methods Thirty-one male rats were randomized to following groups: 1 - control; 2 - L-Nitroarginine methyl ester (L-NAME); 3 - Tamsulosin + L-NAME, 4 Tadalafil+L-NAME; and 5 - Tamsulosin + Tadalafil + L-NAME. At the end of the treatment period (30 days), all animals were submitted to urodynamic study. Results The administration of L-NAME increased the number of non-voiding contractions (NVC) (1.04 ± 0.22), volume threshold (VT) (1.86 ± 0.35), and micturition cycle (MC) (1.34 ± 0.11) compared with control (0.52 ± 0.06, 0.62 ± 0.06, and 0.67 ± 0.30), respectively. The administration of tamsulosin reduced the number of NVC (0.57 ± 0.42) and VT (0.76 ± 0.24 ) compared with L-NAME group. Co-treatment with tadalafil decreased the number of VT (0.85 ± 0.53) and MC (0.76 ± 0.22) compared with L-NAME group. The combination of tamsulosin with tadalafil improved the number of NVC (0.56 ± 0.18), VT (0.97 ± 0.52) and MC (0.68 ± 0.30) compared with L-NAME group. Conclusion In rats with BOO induced by chronic nitric oxide deficiency, tadalafil did not cause impairment in detrusor muscle and seems to have an addictive effect to tamsulosin because the combination decreased non voiding contractions as well the number of micturition cycles.