The role played by the mechanical tissue stress in supporting lymph formation and propulsion in thoracic tissues was studied in deeply anesthetized rats (n = 13) during spontaneous breathing or mechanical ventilation. After arterial and venous catheterization and insertion of an intratracheal cannula, fluorescent dextrans were injected intrapleurally to serve as lymphatic markers. After 2 h, the fluorescent intercostal lymphatics were identified, and the hydraulic pressure in lymphatic vessels (P_lymph) and adjacent interstitial space (P_int) was measured using micropuncture. During spontaneous breathing, end-expiratory P_lymph and corresponding P_int were −2.5 ± 1.1 (SE) and 3.1 ± 0.7 mmHg (P < 0.01), which dropped to −21.1 ± 1.3 and −12.2 ± 1.3 mmHg, respectively, at end inspiration. During mechanical ventilation with air at zero end-expiratory alveolar pressure, P_lymph and P_int were essentially unchanged at end expiration, but, at variance with spontaneous breathing, they increased at end inspiration to 28.1 ± 7.9 and 28.2 ± 6.3 mmHg, respectively. The hydraulic transmural pressure gradient (ΔP_tm = P_lymph − P_int) was in favor of lymph formation throughout the whole respiratory cycle (ΔP_tm = −6.8 ± 1.2 mmHg) during spontaneous breathing but not during mechanical ventilation (ΔP_tm = −1.1 ± 1.8 mmHg). Therefore, data suggest that local tissue stress associated with the active contraction of respiratory muscles is required to support an efficient lymphatic drainage from the thoracic tissues.