Solution culture experiments were made using two subspecies of Cerastium fontanum and Cochlearia pyrenaica ssp. alpina (Bab.) Dalby and Festuca rubra L. of ultramafic and non-ultramafic origin to test: (1) whether Mg and Ni toxicities are likely to exclude non-ultramafic races of the three species; (2) whether tolerance to Mg and Ni toxicities by the ultramafic plants differed markedly among the species; and (3) to use (2) to generate a hypothesis about controlling factors of species distribution on the Meikle Kilrannoch (MK1) ultramafics. The solution cultures were based on soil solution extracts with experimentally varied concentrations of Mg, Ni and Na. The solution with high (7·5 mM) Mg and high (0·7 mg l−1) Ni concentrations simulated the ultramafic soil solution. Care was taken to use a Fe source which did not react with Ni and microelements. Of the non-ultramafic plants, the growth of Cerastium was reduced by high concentrations of Mg but not by Ni, whereas both the Cochlearia and the Festuca suffered severe reduction in biomass production in the presence of high Mg or Ni or both. Of the ultramafic plants, Festuca was tolerant of high Mg and Ni concentrations; Cerastium was less tolerant of high Mg; and Cochlearia had low shoot production in high Ni (although the effect was ameliorated by high Mg). The different interspecific responses of the ultramafic plants to Mg and Ni supported field observations that the three species might occupy microhabitats which differ in Mg and Ni.