The article examines the environmental and legal mechanism of the Ukrainian limitation regulation of pollutant discharges (RPD) into water bodies (WB). The central methodological link of RPD is the «Basin principle of determination of standards of maximum allowable pollutant discharge» (BPDSMAPD). This principle introduced by outdated regulatory & legal and regulatory & technical acts of last century. This basin principle (BP) based on the use of the assimilative possibility (AP) of WB for the environmental & economic purposes of reducing the harmfulness of hazardous substances discharged into WB to the level of maximum admissible concentrations (MAC) in water for various purposes of water use. This RPD method does not meet the requirements for harmonization with modern EU water legislation. The terminological error in the name does not allow differentiate the concept of "basin management principle", which has its roots in EU water legislation, and Ukrainian concept BPDSMAPD. Instead, the term "determination of MAC standards for cascading discharge of P" was proposed. The environmental & legal grounds, justification and consequences of the application of the BPDSMAPD, and requirements for the maximum permissible discharges (MPD) of P in Ukrainian environmental and water legislations identified. Under BP approach the AP phenomenon should be considered as a common beneficial quality of WB for the cascade of P discharges, which is subject to distribution among economic entities in case of special water use. It was determined that in fact the AC has a local (point) not a basin property and is not subject to division. In the environmental and water legislation of Ukraine as and EU, there is no concept of AC, and requirements for the use of AC are not established. Made conclusions that the "right to quota" of AP for water users, on which based the BP, does not exists, and the actual implementation of the BP leads to a violation of the basic legal right to use the same MPC within entire territory of Ukraine. The path to the legal regulation of the cascading discharge of P through the introduction of changes in the Ukrainian water legislation aimed at the organization of joint special water use is proposed.