The role played by 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 hours, the fluorescent intercostal lymphatics were identified and the hydraulic pressure in lymphatic vessels (P_lymph) and adjacent interstitial space (P_int) was measured by micropuncture. During spontaneous breathing, end-expiratory P_lymph and corresponding P_int were - 2.5 ± 1.1 (1 SE) mmHg and 3.1 ± 0.7 mmHg (p < 0.01) and dropped to -21.1 ± 1.3 mmHg 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 mmHg and 28.2 ± 6.3 mmHg, respectively. The hydraulic transmural pressure gradient (P_tm = P_lymph - P_int) was in favour of lymph formation throughout the whole respiratory cycle (P_tm = - 6.8 ± 1.2 mmHg) during spontaneous breathing, but not during mechanical ventilation (P_tm = - 1.1 ± 1.8 mmHg). Therefore, data suggest that local tissue stress associated to the active contraction of respiratory muscles is required to support an efficient lymphatic drainage from the thoracic tissues.