The relationship between locomotion and aspiration breathing was investigated in the lizards Iguana iguana, Ctenosaura similis, Varanus exanthematicus and Varanus salvator, and the quail Coturnix coturnix. Respiratory air-flow during walking and running on a 7.3 m track or on a treadmill was measured with a bidirectional flow meter attached to one nostril. In all four species of lizards, lung ventilation drops markedly during locomotion. Tidal volume decreases as speed increases, often by more than an order of magnitude at intermediate and high speeds, and the rate of decline is most pronounced at the lowest speeds. Minute ventilation peaks at or before the reported maximum aerobic speed and decreases at higher speeds. In contrast, quail increase their minute ventilation during running. Several observations support the hypothesis that the aspiration of lizards is mechanically constrained by locomotion which employs lateral vertebral bending and sprawling posture. 1. Minute ventilation decreases as running speed increases. 2. Disruption of ventilation is temporally coincident with the locomotor movements. 3. During running the largest breaths correspond to the strides of longest duration or to brief pauses in the locomotor movements.