Lithium phthalocyanine (LiPc) is a prototype of another generation of synthetic, metallic-organic, paramagnetic crystallites that appear very useful for in vitro and in vivo electron paramagnetic resonance oximetry. The peak-to-peak line width of the electron paramagnetic resonance spectrum of LiPc is a linear function ofthe partial pressure ofoxygen (PO2); this linear relation is independent of the medium surrounding the LiPc. It has an extremely exchange-narrowed spectrum (peak-to-peak line width = 14 mG in the absence of 02). Physicochemically LiPc is very stable; its response to pO2 does not change with conditions and environments (e.g., pH, temperature, redox conditions) likely to occur in viable biological systems. These characteristics provide the sensitivity, accuracy, and range to measure physiologically and pathologically pertinent 02 tensions (0.1-50 mmHg; 1 mmHg = 133 Pa). The application of LiPc in biological systems is demonstrated in measurements of PO2 in vivo in the heart, brain, and kidney of rats.The purpose of this article is to describe a technology based on electron paramagnetic resonance (EPR or equivalently, electron spin resonance, ESR) that can significantly improve the ability to measure the partial pressure of 02 (PO2) under biologically pertinent conditions in vitro, in vivo, and potentially in human subjects. This article focuses on a prototype ofa class of crystalline paramagnetic probes, lithium phthalocyanine (LiPc), and aims at providing sufficient detail to facilitate the use of these probes in viable biological systems. The critical capabilities of this technology are the ability to measure PO2 at the levels (usually <40 mmHg and can be as low as 0.1 mmHg; 1 mmHg = 133 Pa) and sites (e.g., in tissues in vivo and inside cells) needed to understand biological processes.