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Chlorine-36 is a radionuclide with a half-life of 3.01 x 10(5)a. Most Cl-36 in the hydrosphere originates from cosmic radiation interacting with atmospheric gases. Large amounts were also produced by testing thermonuclear devices during 1952-58. Because the monovalent anion, chloride, is the most common form of chlorine found in the hydrosphere and because it is extremely mobile in aqueous systems, analyses of both total Cl- as well as 36Cl have been important in numerous hydrologic studies. In almost all applications of 36Cl, knowledge of the initial, or pre-anthropogenic, levels of Cl-36 is useful, as well as essential in some cases. Standard approaches to the determination of initial values have been to: (a) calculate the theoretical cosmogenic production and fallout, which varies according to latitude; (b) measure Cl-36 in present-day precipitation and assume that anthropogenic components can be neglected; (c) assume that shallow groundwater retains a record of the initial concentration; (d) extract Cl-36 from vertical depth profiles in desert soils; (e) recover Cl-36 from cores of glacial ice; and (f) calculate subsurface production of Cl-36 for water that has been isolated from the atmosphere for more than one million years. The initial value from soil profiles and ice cores is taken as the value that occurs directly below the depth of the easily defined bomb peak. All six methods have serious weaknesses. Complicating factors include Cl-36 concentrations not related to cosmogenic sources, changes in cosmogenic production with time, mixed sources of chloride in groundwater, melting and refreezing of water in glaciers, and seasonal groundwater recharge that does not contain average year-long concentrations of Cl-36.