Using a three-dimensional Fourier transform approach, proton nuclear magnetic resonance (NMR) chemical shift images have been obtained in vivo for the first time. At a proton resonance frequency of 61.5 MHz, chemical shift-resolved images of simple phantoms indicate that a spectral resolution of 0.7 parts per million (ppm) is readily achievable at all locations within the image matrix, even when using a magnet not specifically designed for chemical shift spectroscopy. In vivo images of the human forearm and of a cat head yield separable signals from water and lipid protons. However, using simple radiofrequency pulse sequences, our data show that relatively little signal originates from membrane lipids (e.g., myelin) in the brain. The measurement of magnetic susceptibility using this technique is also demonstrated. While helping to elucidate the genesis of the NMR response in complex biological systems, this methodology also has potential applications in medical diagnosis. The technique is also applicable to the chemical shift imaging of other nuclei; for example, phosphorus (P-31).