The effect of copper on root growth, K+ efflux and short-term copper uptake by three isogenic genotypes of Mimulus guttatus Fischer ex DC. differing only in their copper tolerance gene is studied. The tolerant homozygotes do not differ in any way from the heterozygotes. The non-tolerant homozygotes show greater root inhibition, greater K+ efflux, and greater short-term Cu2+ uptake. The difference in the uptake parameters is best described by a greater linear diffusion term in the non-tolerant genotypes. These results suggest that copper damages the cell membrane, and that this damage leads to an increase in the leakiness of the membrane, leading to both greater K+ efflux and Cu2+ influx via diffusion. This suggests that the primary copper tolerance mechanism probably resides in the cell membrane. The rapidity with which the differences between the genotypes is established suggests that the differences are constitutive. It is argued that these results provide further evidence against the involvement of phytochelatins in the primary tolerance mechanism.