The ultramafic sites at Meikle Kilrannoch, Scotland, have small sparsely colonized patches with soils which have highly toxic (to many non-indigenous plants) concentrations of Mg and Ni. These toxic metals are unlikely to be the main cause of the open vegetation as the indigenous plants are at least partially tolerant of them. The hypothesis that low. soil nutrients were a likely cause of the open vegetation was tested by a fertilization experiment in which major nutrients and Ca were added in August 1991 and July 1992 to replicated quadrats which initially had 5.3-6.6% plant cover. There was a large increase in plant cover on major nutrient addition. The increase in rosette size, flowering, seed production, and recruitment of the dominant semelparous Cochlearia pyrenaica DC. ssp. alpina (Bab.) Dalby was studied in detail. The increased growth of Cochlearia (and some other native species at the site) on nutrient addition strengthens the argument that a major nutrient deficiency rather than a metal toxicity limits plant growth at Meikle Kilrannoch and shows that at least some stress tolerators respond with rapid growth and reproduction when nutrient limitation is removed. A small-scale experiment set up in June 1993, in which the major nutrients were added separately, suggested that P and not N, or K was the limiting element. The effects of P seem to be unrelated to any possible reduction in the availability of the toxic ions Mg2+and Ni2+.