Male and in some cases female rats were exposed to an oil fog generated by flash vaporization and subsequent condensation of light-weight lubricating oil. Exposures were for 3.5 h/d, 4d/wk for 13 wk. Males were exposed at concentrations of 1.5, 0.5, 0.2 or 0.0 mg/l (1500, 500, 200, and 0 mg/m3) and a particle size of approximately 1 micron (mass median aerodynamic diameter). A number of biologic endpoints were assessed the day after the last exposure and, in some cases, after a 4 wk recovery period. Effects of 1.5 mg/l on male and female rats were compared. Diffuse accumulation of macrophages in the alveoli was observed in all oil fog exposed groups. The degree of severity was concentration dependent. Histopathologic changes were more prominent in males than in females and represented the most notable gender-related differences. Histologic effects observed one day and 4 wk post exposure were similar. Minimal histopathologic changes and minimal increase in lavage fluid protein were the only effects observed at the 0.2 mg/l exposure level. There was a significant increase in lavage fluid protein, percent lavagable polymorphonuclear leukocytes and lung wet and dry weight following exposure to both 0.5 and 1.5 mg/l. At the highest exposure concentration effects on lung weights were still evident 4 wk post exposure. Pulmonary function endpoints including total lung capacity, vital capacity, residual volume, diffusing capacity to CO, compliance, and end expiratory volume (EEV) were unaffected by oil fog exposure with the exception of EEV in males exposed at the 1.5 mg/l level. All of the changes observed following oil fog exposure were consistent with a mild inflammatory edema.