We have observed the structure of the rapid variable radio source BL Lac (VRO 42.22.01) using long baseline interferometer systems with baselines up to 266 million wavelengths. Despite large variations in the total flux and in the overall size of this source, it has maintained an elongated brightness distribution, and the direction of elongation has not changed during the 1.3 years of observation. No simple model of stationary variable components or of separating, evolving components appears to fit all the data For this source. In particular, it apparently cannot be explained in terms of a stationary brightness distribution with a single variable component of very small angular size. The peak brightness temperature of VRO 42.22.01 is in excess of 5x 10/sup 12/. K at 11 cm wavelength, a value close to the limit set by inverse Compton scattering.