We studied the flow-impeding characteristics of the exhalation valves and PEEP attachments commonly used in mechanical ventilation. To characterize these devices, the pressure difference across each mechanism was measured at a series of constant flows (5 to 160 L/min), and resistance-related energy dissipation was measured using mechanical models of passive and active exhalation. At ambient end-expiratory pressure, an inflatable diaphragm (mushroom) design commonly used to valve exhalation presented resistance comparable to that of an endotracheal tube with an internal diameter of 5 mm. The valve's energy dissipation increased further as PEEP was applied. By comparison, the servo-actuated scissor valve we tested presented less resistance during the passive deflation experiment but impeded the early phase of active exhalation. Spring-loaded PEEP attachments were prohibitively resistive in comparison with alternative methods using an underwater tube, a water column, a weighted spirometer, or an inflatable diaphragm to raise end-expiratory pressure. We conclude that the exhalation valves and PEEP attachments currently available for clinical use present significant impedance to air flow. Such resistance within the exhalation pathway may be clinically important for patients supported by mechanical ventilation during the hyperpneic or weaning phases of their illness.