The role of gibberellins in regulating the growth of tomato roots was investigated by comparing various cellular parameters in cultured roots of the gibberellin-deficient mutant gib-l/gib-l with those in roots of the near-isogenic wild-type. In addition, wild-type roots treated with 0·1 μM 2S,3S paclobutrazol, an inhibitor of gibberellin biosynthesis, and mutant roots treated with 0·1 μM GA3 were also compared: the former roots constitute a phenocopy of the mutant, whereas the latter roots appear to be ‘normalized’ and similar to wild-type. The elongation of mutant and phenocopied roots were similar, their maximum elongation rates being about half or two-thirds that of wild-type or GA3-treated mutant roots, respectively. These rates were interpreted in terms of the numbers and lengths of cells within the meristematic and non-meristematic portions of the elongation zone. Mean meristem length tended to be shorter in both the mutant and the 2S,3S paclobutrazol-treated wild-type roots than in the other two types of root. A major difference between the two pairs of mutant and normal roots was their mean final cell lengths: mean lengths of cortical cells of the mutant and 2S,3S paclobutrazol-treated roots were, respectively, 39% and 25% shorter than the mean length of wild-type roots. Final cell length in the GA3-treated mutant roots were similar to wild-type. By contrast, the diameters of mature cortical cells of the mutant and phenocopy were about 20% greater than the diameters of equivalent wild-type or ‘normalized’ mutant cells. The mean volumes of cortical cells in all four types of roots showed no significant differences. Knowledge of the distribution of cortical cell lengths, widths and volumes along the root axis, together with information about the rate of root elongation, permitted comparisons of the relative elemental growth rates of each of these three cellular parameters. The available evidence suggests that the level of endogenous gibberellins in mutant roots is lower than in wild-type roots. The present results, therefore, suggest that endogenous gibberellins are necessary for normal growth of cultured tomato roots and that they regulate the relative amounts of growth at the longitudinal and transverse walls of the cells which, in turn, affects the shape of the elongating cells.