The mechanistic basis of Al rhizotoxicity was investigated through a time-based analysis of the growth reactions seen in hydroponically-grown roots of wheat isolines (IS and IT) differing in Al tolerance, when exposed to 75 μM Al (pH 4.1) for times ranging between 2 minutes and 48 h. Notwithstanding genotypic differences in the timing and the magnitude of the reactions, it was clear that the root growth adjustments made in response to Al involved three phases, 1) the primary response (IS and IT ≤ 60 min) which produced a net increase in root extension growth, 2) an intermediate period of growth inhibition (IS, 6 h; IT, 24 h.) and 3) recovery (IT 48 h) which included growth stimulation. Close relationships also emerged between the time-dependent, Al-induced increases and decreases in root extension growth and the volume (length) of the root cap and the length of root epidermal cells which suggested that cap volume and root epidermal cell extensibility may also be components of the Al response network. These findings are discussed with a view to developing a coherent theory of Al rhizo- toxicity/root growth control. Genotypic similarities and differences in the root growth responses shown to Al are used to distinguish between Al escape strategies (stress avoidance, IT and IS) and the longer term activation of genetically controlled mechanisms of Al tolerance (IT) involved in the recovery from Al.