To investigate apoptosis as a mechanism of sulfur mustard (SM) inhalation injury in animals, we studied different caspases (caspase-8, -9, -3 and -6) in the lungs from a ventilated rat SM aerosol inhalation model. SM activated all four caspases in cells obtained from bronchoalveolar lavage fluid (BALF) as early as 6 hr after exposure. Caspase-8, which is known to initiate the extrinsic Fas-mediated pathway of apoptosis, was increased 5-fold between 6 to 24 hr, decreasing to the unexposed-control level at 48 hr. The initiator, caspase-9, in the intrinsic mitochondrial pathway of apoptosis as well as the executioner caspases, caspase-3 and -6, all peaked (p<0.01) at 24 hr; caspase-3 and -6 remained elevated, but caspase-9 decreased to unexposed-control level at 48 hr. To study further the Fas pathway, we examined soluble as well as membrane-bound Fas ligand (sFas-L, mFas-L, respectively) and Fas receptor (Fas-R) in both BALF cells and BALF. SFas-L increased significantly at 24 hr after SM exposure in both BALF cells (p<0.01) and BALF (p<0.05). However, mFas-L increased only in BALF cells between 24 to 48 hr (p<0.1, <0.001, respectively). Fas-R increased only in BALF cells by 6 hr (p<0.01) after SM exposure. Apoptosis in SM-inhaled rat lung specimens was also confirmed by both immunohistochemical staining using cleaved caspse-3 and -9 antibodies and TUNEL staining as early as 6 hr in the proximal trachea and bronchi, but not before 48 hr in distal airways. These findings suggest pathogenic mechanisms at the cellular and molecular levels and logical therapeutic target(s) for SM inhalation injury in animals.