A nuclear magnetic resonance (NMR) machine capable of producing tomographic sections of the whole human body in vivo has been constructed. This system is based on a four coil, air-core electromagnet producing a field of 0.04 T which corresponds to a proton NMR frequency of 1.7 MHz. The images are produced line by line using a selective excitation technique. Magnetic field gradients up to about 5 mT/m are employed. Electronic subsystems are described here including a radiofrequency (RF) amplitude feedback circuit, an RF power amplifier, a transmit/receive switch, a receiver pre-amplifier and gradient coil drivers. For a single scan through a 103 mm3 sample of human muscle tissue in vivo, the measured proton density uncertainty is 24% and the spin-lattice relaxation time (T1) uncertainty is 74%. Phantom images using CuSO4 solution and in vivo sections through human chest, thighs and head are presented. T1 measurements of human muscle, liver and brain tissue in vivo give results which agree well with T1 values for corresponding rabbit tissues measured in vitro.