A cadmium tolerant and a non-tolerant population of Holcus lanatus were subjected to a pretreatment with either no or low (0·2 μg Cd ml−1) concentrations of cadmium prior to normal tolerance testing in 1·0 μg Cd ml−1 solution. When pretreated roots were not removed prior to the tolerance test, the cadmium pretreatment resulted in significantly greater root growth of both populations in the test solution relative to plants given no cadmium pretreatment. Repeating the tolerance test on plants which had had their pretreated roots excised prior to testing showed no significant effect of any pretreatment on subsequent root growth in either population. There was an overall difference between the two populations in terms of their response to pretreatment and test concentrations in that when exposed to cadmium, plants of the tolerant population grew significantly better than those of the non-tolerant population. The data presented provide evidence for a stimulation of cadmium tolerance following low-level exposure of roots to cadmium. The mechanisms associated with these effects are not clear although cadmium tolerance in animals is known to show similar trends.