Scale effects on hydraulic conductivity (i.e. increase in the value of K with increasing scale of measurement) have been reported in the literature. Here, we summarize extensive tests performed at El Berrocal, a granitic site in Central Spain, to study whether such effects indeed appear. In fact, median hydraulic conductivities clearly increase with measurement scale. We argue, however, that large scale measurements may be biased toward high K values because they are performed in the most permeable zones, where pumping rates can be high. This is supported by the fact that cross-hole transmissivities and hydraulic conductivities consistently fall below the corresponding values at the pumping interval. ¿ American Geophysical Union 1995