Ten patients were started on treatment with large doses of vitamin D during the period of the study. The rate of rise of plasma 25-OHD was followed during treatment. The incremental rise in 25-OHD was calculated at the end of the first week of treatment in terms of dose per unit body weight. The rate of rise of plasma 25-OHD level was highly correlated with the dose used. Plasma 25-OHD levels after one week's treatment were only 15-20% of the expected steady-state level on the same dosage. The importance of a high priming dose when a rapid response is needed is thus emphasised.The availability of a precise and convenient assay for 25-hydroxyvitamin D has yielded important information bearing on many aspects of metabolic bone disease (Haddad and Stamp, 1974;Offermann et al., 1974;Preece et al., 1975

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