To understand the behavior of many NMR expts., it is important to det. the spatial distribution of the B1 field. In this paper, we show how this distribution can be mapped independently of spin d., coil responsiveness, and nonlinearities of the B0 field gradients. As a byproduct we obtain a map of the (possibly nonlinear) spatial variation of the B0 field gradients used in the imaging procedure. (c) 1999 Academic Press. [on SciFinder (R)]