Studies of response to low doses of edematogenic gases have been hampered by the insensitivity and the nonquantitative nature of the major indicators of response. A new and more sensitive indicator, the recovery of 131I-albumin from the alveolar spaces six hours after its intravenous injection in rats, has been applied. Significantly increased albumin recovery was found for all concentrations of ozone at and above 0.5 ppm. No wet weight changes were noted below 2.5 ppm ozone, and there was no consistent histologic finding except for slight sloughing of bronchial epithelium at 2.5 ppm. Application of these methods to studies of steroid effects revealed increased sensitivity to ozone following administration of methylprednisolone sodium succinate. In addition, animals treated with steroids prior to exposure to 0.25 ppm ozone became tolerant to subsequent ozone challenge, whereas animals given preexposure but no steroids did not.