The Bet Shean and Harod valleys are regional recipients and mixing zones for groundwaters draining from a regional multiple-aquifer system comprising two different carbonate and two basalt aquifers as well as deep-seated reservoirs of confined brines. This paper, the first of two, describes the hydrochemistry of the groundwater bodies related to this multiple aquifer system. The paper deals with the chemical composition of rain falling on natural recharge areas, the chemical contribution of the aquifer rocks and the chemical evolution of groundwaters in the upper flow courses of all aquifers of this regional system. The methodology applied is based on the determination of major dissolved ions and on the examination of their ratios and changes along the upper flow paths. Rain water falling on natural recharge areas has an average chlorinity of 12mgl −1Cl − and a calcium bicarbonate composition caused by dust-borne terrigenous material. This rain-water salinity decreases with distance from the sea-shore. Fresh groundwaters flowing through the different aquifers may be identified and differentiated by their characteristic salinity levels and ionic ratios. Groundwaters flowing through carbonate aquifers are identified by different salinity levels and by distinct rMg/rCa ratios reflecting dolomite-calcite ratios in aquiferr rocks. The groundwaters of the two basalt aquifers have typical cation assemblages and high Na + and Mg 2+ concentrations, far in excess of those typical of the recharging rainfall. Groundwater in the upper flow courses of all aquifers investigated is deficient in SO 2− 4 and K + relative to recharging rain water.