The assessment of lung mechanical function in small animals, particularly mice, is essential for investigations into the pathophysiology of pulmonary disease. The most accurate and specific methods for making this assessment are highly invasive, and so provide data of questionable relevance to normality. By contrast, current noninvasive methods based on unrestrained plethysmography have no direct link to the mechanical properties of the lung. There is thus a need for a completely noninvasive method for determining lung mechanical function in small animals. In the present study, we demonstrate an extension of unrestrained plethysmography in which changes in lung volume are estimated via orthogonal video imaging of the thorax. These estimates are combined with the pressure swings recorded as mice breathe inside a heated and humidified chamber to yield an estimate of specific airways resistance (sR_aw). We used this new technique, which we term Unrestrained Video-Assisted Plethysmography (UVAP), to measure sR_aw in 11 BALB/c mice exposed to aerosols of saline, methacholine, and albuterol, and obtained mean values of 0.71, 1.23 and 1.10 cmH₂O.s, respectively. Mean breathing frequency was 4.3, 3.4 and 3.6 breaths/s, respectively, while the corresponding mean tidal volumes were 0.36, 0.44 and 0.37 ml, respectively. We conclude that UVAP, a noninvasive method, is able to provide usefully accurate estimates of sR_aw and breathing pattern parameters in mice.