The diffusion coefficient is an important parameter for characterizing the state of water in biological systems. A nuclear magnetic resonance method of diffusion measurement by the pulsed field gradient, technique is described. A 9-T-18 spin-echo sequence is used with one gradient pulse located on each side of the 18 rf pulse. Information on restricted diffusion is obtained by varying the measuring time, which is determined by the separation of the gradient pulse pair. Measuring times greater than 2 msec are possible with the present apparatus. Maximum magnetic field gradients of 11 gauss/cm are applied to cylindrical sample volumes of 1 cm diameter and .5 cm in height. This permits the measurement of diffusion coefficients as low as 1 cm /sec. Details of the electronic hardware and gradient coil construction and calibration are given. The measurement process is software supported and includes digital data acquisition.