Steady-state and pulse n.m.r. techniques have been applied in a study of water in the cytoplasm of slime mould plasmodia (Ph;ysarum polycephalum). The former method has been used to confirm that the signal detected was from protons in water molecules, and to estimate the fraction of the total water in the sample that was contributing to the observed signal. Pulse techniques have enabled direct measurement of the self-diffusion coefficient D of the bulk water in the cytoplasm, and the proton spin-lattice and spin-spin relaxation times Tl and T2 respectively. The measurements of D can be accounted for if most of the water is in a "free" state, similar to water in a dilute ionic solution, but with a lower value of D due to the obstruction effect of macromolecules and cytoplasmic structures. The values of Tl and T2 indicate that a small "bound" fraction of the water molecules has more restricted motion. The assumption of a two-state model, with exchange of water molecules between "free" and "bound" phases in a time ~ 10-3 sec, yields a representative correlation time T "'" 10-8 sec for the bound fraction. This model is the simplest compatible with all the above results. The underlying assumptions, the extent to which it is likely to be an approximation, and the implications regarding some theories of cellular functions are discussed. Similar results have also been obtained from samples of toad leg muscle, ceils from the meristematic region of pea roots, and from agar and gelatin gels.