Increasing awareness of the vast amount of information that can be obtained from intrinsic and extrinsic probes of membrane phenomena has led to a wide-ranging survey of the properties of both types of probe. Intrinsic probes of photosynthetic membranes are provided by the carotenoid pigments. The carotenoids are assumed to be located in the lipid phase of the membrane close to chlorophyll, since they act as accessory light-harvesting pigments. Isolated reaction center preparations from photosynthetic bacteria usually contain tightly bound carotenoids, which further supports the concept of a close association between them and the photosynthetic unit. The spectral and kinetic properties of these pigments respond to several parameters: light, oxygen, ATP, and inorganic pyrophosphate (PPi). In single-flash illumination experiments, the rate of decay of the membrane charge is assumed to depict the degree of coupling of the chromatophore or chloroplast membrane. The origin of the carotenoid absorbance change has been much debated and recently it has been shown to coincide with the formation of a diffusion potential in chromatophore membranes from Rhodopseudomonas spheroides (Jackson and Crofts, 1969, 1971; Crofts, 1974). In plant chloroplasts, the corresponding absorbance change at 518 nm has been stated by Witt (1972) to indicate the formation of a transmembrane electric field. When induced in the dark by energizing conditions, or by illumination, the steady state response of the pigments is abolished by uncouplers but the response to a single flash is unaltered in rise time and extent, even though the decay time is accelerated.