The activity of developing agents in the physical developers, which have equal silver and redox potentials but contain a different complexing agent, has been investigated by means of post-fixation physical development. Kodak CD-3, Metol and hydroquinone has been used as a developing agent in the developers. For all developing agents examined, the use of thiocyanate as a complexing agent gives larger development rate ( dAg/dt ) at the initial stage than the use of thiosulfate. We assume that this is because;(1) adsorption of S₂O₃^2- to silver nucleous is stronger than that of SCN-, and adsorption of these ions decreases the activity of development center:(2) Ag (SCN) _n (^n-1)- is reduced more easily than Ag (S₂O₃) n ^(2n-1)-. For the developers of same complexing agent, development rate ( dAg/dt ) at initial stage is the order, the use of Metol, Kodak CD-3, hydroquinone. When Metol is used, development rate of the developer containing thiocyanate as a complexing agent decreases with development proceeding and the developer is exhaused rapidly. But the rate of the developer containing thiosulfate increases with developing time, and to our surprise, the used developer is more active than the fresh one. It is suggested that the increase of development activity of the used developer arises from producing active developing agent (e.g. powerful adsorbing developer thiosulfonate) by the reaction of oxidized developing agent with thiosulfate.