In response to the need to better describe spatial variations in permeability, we designed and built a new portable field air permeameter for use on rocks in outcrop and core. In this instrument, a chamber containing a small volume of air in contact with the rock is suddenly increased in volume creating a vacuum, causing air to flow from the rock into the chamber. The instantaneous chamber volume and the air pressure within it were monitored. We evaluated a theory to allow interpretation of these data for the rock permeability. The theory required knowledge of some difficult-to-determine geometric constants. This difficulty was circumvented in practice through a calibration procedure. We described the theory for the design and construction of this instrument and reviewed some successful uses in field-scale studies of rock permeability heterogeneity.