We used a video digitizer system to measure the kinetics of promotion and inhibition of root elongation by applied auxin in roots of intact seedlings of Arabidopsis thaliana. We determined the dose response relationship for auxin action on elongation in roots of wild type seedlings and seedlings of four mutants with altered auxin responsiveness. Roots of all four mutants maintained more rapid rates of elongation in the presence of inhibitory concentrations of auxin than did roots of the wild type. The latent period preceding the change in elongation rate after auxin application was the same in the mutants as in the wild type indicating that the differences in the auxin response between the wild type and the mutants are not due to differences in hormone uptake. Low concentrations of auxin induced substantial promotion of root elongation in the wild type and in the mutants aux1-7 and axr2 but not in axr1-3 or axr1-12. The mutants aux1-7 and axr2 showed no gravitropic curvature when stimulated at 90°. The mutants axr1-3 and axr1-12 exhibited rates of gravitropism intermediate between the non-responsive mutants and the wild type. For these intermediate mutants there was an inverse relationship between growth rate and rate of gravitropic curvature.