We compared the response of root elongation to a wide range of pH for Lupinus angustifolius cv. Yandee and Pisum sativum cv. Dundale grown in buffered and unbuffered nutrient solutions. Root elongation of peas was not affected by pH from 4.5 to 8.0, whereas root elongation of lupins was markedly decreased by increasing pH from 5.5 to 6.0 and further decreased when pH was increased to 8.0; the pH optimum for lupin root elongation was between 5.0 to 5.5. The decreased lupin root growth was caused by a decreased cell elongation rather than cell division. This effect of high pH occurred within one hour and was readily reversible. Additionally, cortical cells of lupin roots exposed to pH 6.5 and 8.0 expanded spherically rather than longitudinally, resulting in an increased root diameter. Decreased root elongation of lupins grown at high pH was greater in buffered solutions than in unbuffered solutions.