Constant-head permeability (hydraulic conductivity) tests were performed on samples of a compacted clay soil using a 0.914 by 0.914 by 0.457-m (3 by 3 by 1.5-ft) large-scale, double-ring, rigid-wall permeameter. A naturally occurring silty clay soil was used for the permeability tests. The soil was separated into five different fractions representing five different ranges in precompaction clod sizes. Soil from each of the soil fractions was used for soil specimens. The soil for the large-scale permeameter was compacted in two 7.62-cm (3-in) lifts. Small-scale, constant-head permeability tests also were performed on soil specimens compacted into standard Proctor molds (9.44 × 10-4 m3). Comparison of the results from the two different scales of permeameters indicated that, in all cases, the permeability for a given soil fraction was higher in the large-scale permeameter than it was in the small-scale permeameter. In addition, the permeability for all soil fractions measured in the large-scale permeameter ranged from 0.6 to 2.4 orders of magnitude higher than the value measured in the small-scale permeameter. As a result of the permeability tests performed in this study, there appears to be a scale effect associated with laboratory permeability testing, especially when a significant proportion of the soil being tested consists of precompaction clod sizes which are large relative to the size of the permeameter. The scale effect in this study is thought to be due to the relationship between the compactive effort and the different degrees of confinement associated with the different scales of permeameters. The implication of the study is that a more realistic evaluation of the fieldmeasured permeability of a compacted clay soil may be possible in the laboratory if the permeameter is sufficiently large to test a representative sample of soil.