Vitamin D metabolites were measured on admission in eight patients intoxicated with ergocalciferol (serum calcium 3.01-4.05 mmol/l) and also during the subsequent 2 months in six of the eight. Serum concentrations of 25-hydroxyergocalciferol, on admission, were grossly elevated in all patients (range 583-1843 nmol/l). Serum calcium concentration was related significantly only to the concentration of 25-hydroxyergocalciferol (P = 0.003). Concentrations of 25-hydroxyergocalciferol in serum were significantly related to those of calciferol (P = 0.004). Elevated initial concentrations of 1,25-dihydroxycalciferol, mainly as 1,25-dihydroxyergocalciferol, were found in seven of the eight patients (range 179-313 pmol/l). It is suggested that the hypercalcaemia in these patients may be explained by the action of 25-hydroxyergocalciferol at high concentration in competing for 1,25-dihydroxycalciferol receptors, thus exerting a biological effect per se, and also by increasing the synthesis of 1,25-dihydroxycalciferol through a mass-action effect on the renal 1 alpha-hydroxylase.