An in-vitro lung model and a volume ventilator were used to evaluate the delivery of aerosolized albuterol through an infant ventilator circuit. We compared the following: continuous nebulization (CNA) and intermittent nebulization (INA); various nebulizer gas flows, 5.0, 6.5,and 8.0 L/min; and duty cycle of 33% and 50%. The efficiency and consistency of aerosol delivery by metered-dose inhaler (MDI) with four different spacer devices and by nebulizer positioned at the manifold and at the same position as the MDI were also evaluated. A volume ventilator (Servo 900B) was used with settings selected to reflect those of a moderately to severely ill 4-kg infant. A 3.5-mm endotracheal tube was used in all experiments. A specific type of nebulizer used (Airlife Misty Neb; Baxter; Valencia, Calif) and several spacers were studied (Aerochamber and Aerovent, Monaghan Medical Corporation in Plattsburgh, NY [corrected]; ACE, Diemolding Healthcare Division in Canastota, NY [corrected]; and an in-line MDI adapter, Instrumentation Industries Inc, Pittsburgh). CNA delivered significantly more aerosol to the lung model (4.8 +/- 0.6% of the starting dose) than INA (3.8 +/- 0.3%; p<0.01). There was a significant stepwise decrease in aerosol delivery with increasing nebulizer flow (4.8 +/- 1.3% at 5.0 L/min; 3.7 +/- 1.1% at 6.5 L/min; and 2.7 +/- 1.1% at 8.0 L/min). Increasing duty cycle did not significantly affeet delivery. Overall the spacers with MDI were more efficient than the nebulizer in either position delivering about twice the percentage of the starting dose than the nebulizers. All modes of delivery, except the Aerochamber, demonstrated a marked degree of variability. Most of the starting dose of albuterol either remained in the nebulizer (30.4 +/- 6.0% at 5.0 L/min and 25.3 +/- 4.1% at 8.0 L/min) or was deposited in the inspiratory tubing (34.7 +/- 0.7% at 5.0 L/min and 43.7+/- 4.9% at 8.0 L/min) in our system. In conclusion, we have confirmed that aerosol delivery depends on the mode of delivery and the operating conditions. Although delivery with an MDI and spacer is more efficient than a nebulizer, both methods may produce high variability depending on the method or spacer used.