The different tidal volumes and frequencies of high-frequency ventilation (HFV) compared with conventional mechanical ventilation (CMV) may have different effects on lung mechanics. To test this hypothesis, we compared the effects of 3 h of HFV and CMV on total lung capacity (TLC), functional residual capacity (FRC), the shape of the pressure-volume (PV) curve (%V10), and dynamic compliance (Cdyn), as well as venous admixture and alveolar-arterial O2 gradient. We studied a total of 12 dogs at lung inflations equivalent to 15 cmH2O positive end-expiratory pressure (PEEP) (group I) and 8 dogs at lung inflations equivalent to 0 cmH2O PEEP (group II). For CMV, we used a standard-volume ventilator at a mean tidal volume of 13.8 ml/kg. For HFV, we used an oscillator-type ventilator at 15 Hz and an average tidal volume of 4.3 ml/kg. Our results showed that ventilation with 3 h of PEEP raised lung volume, and lung volumes on HFV were higher than those on CMV in both groups. Specifically, in group I, the volume during ventilation rose on both CMV (150 ml) and HFV (250 ml). These volume changes persisted beyond the ventilation period, such that TLC was unchanged on CMV but had risen 200 ml on HFV. FRC also rose 200 and 300 ml after HFV and CMV, respectively. In group II, the volume during ventilation fell 100 ml on CMV and rose slightly (40 ml) on HFV. TLC and FRC both tended to fall more on CMV.(ABSTRACT TRUNCATED AT 250 WORDS)