ABSTRACT: Hydraulic data statistics on the specific capacity values of pumping tests could be extracted for 6309 boreholes, hydraulically testing aquifers in pre-Quaternary rocks, from a total of 22,922 wells and hydrogeological boreholes maintained in a database by Geofond reports’ archive of the State Geological Institute of Dionýz Štúr – Geological Survey of the Slovak Republic. The transmissivity index Y data used in this paper was derived from specific capacity by the described, relatively complex re-interpretation process. The main principle therein is based on processing the standard specific yield of the pumped well, thus providing the discharge at the first drawdown metre. The comparison of the resulting mean regional values of the hydraulic indexes Y for these aquifers / aquitards is presented in table form, since 125 general hydrogeological types were identified in pre-Quaternary rocks outcropping on Slovak Republic territory. An interesting feature was found when comparing individual basic aquifer types (Neogene, Neogene Volcanic, Paleogene and Clippen Belt, Mesozoic, Crystalline and Paleozoic) with 30 evaluated boreholes and wells: nearly 95 % of all mean values of these individual datasets were found in the interval of 4.5 to 6.0. According to this, mean transmissivity values of pre-Quaternary rocks are distributed in a relatively narrow interval of one and a half order of magnitude.