A novel magnetic field and radio frequency (1.7 MHz) pulse sequence is described for a whole body n.m.r. imaging machine under construction. Selective excitation is used to obtain signals from successive lines of proton spins (water) across the body to build up an image of a transverse section. The images display spin concentration and spin-lattice relaxation time, T<latex>$_1$</latex>, separately. For a 50% change in T<latex>$_1$</latex> to be discerned in the human trunk, a spatial resolution of 2 cm<latex>$^3$</latex> is expected for a 2 min scan and 0.5 cm<latex>$^3$</latex> for a 30 min scan. Very preliminary images at the present incomplete stage of development show the geometrical accuracy and T<latex>$_1$</latex> discrimination: an in vivo image demonstrates some of the difficulties to be overcome. In vitro measurements of normal rabbit tissue samples have been made at 24 MHz to map the T<latex>$_1$</latex> distributions that can be expected from normal subjects. The transposition of this information from rabbit to man, and from 24 MHz to 2.5 MHz have been checked and the comparison shown to be meaningful. Of pathological samples, human breast tumour and human liver metastases offer a good contrast to their surrounding tissue, and an experimental investigation has shown that tissue immediately surrounding a tumour also has an elevated T<latex>$_1$</latex> value. A wide range of abnormalities that are associated with abnormal fluid formation in the body may be amenable to imaging by the n.m.r. technique. Potential hazards are believed to be small in the present generation of equipment.