The effect of breathing pattern and level of ventilation on fluid filtration in the lung under edema forming conditions was studied in an in situ left upper lobe (LUL) canine preparation. LUL weight was continuously monitored. In Group 1, rate of edema formation (delta W/delta t) was measured In seven dogs at two vascular pressures (35 and 45 mm Hg) while the LUL was randomly ventilated under six conditions. At equivalent vascular pressures and mean airway pressures (Paw) (and hence, mean operating lung volume), increasing respiratory frequency (f) enhanced delta W/delta t. This was reversed when minute ventilation (VE) was returned to baseline by reducing tidal volume (VT), even when Paw were matched to baseline. Increasing VT also enhanced delta W/delta t whether VE was increased (J Appl Physiol 1988; 64:1900) or not (present study) and whether Paw was matched to baseline. In Group 2 delta W/delta t was measured at fixed VT and f while inspiratory/expiratory time ratio (TI/TE) was switched from 1:1 to 1:6. Shortening inspiratory time by increasing inspiratory flow rate had no effect on delta W/delta t. We conclude that increasing VE, whether by raising VT or f, promotes greater edema formation by mechanisms that are independent of vascular pressure or operating lung volume. Increasing VT appears to have an additional adverse effect over and above that of increased VE.