We investigated the effect of oral high-dose cholecalciferol on plasma and adipose tissue cholecalciferol and its subsequent release, and on plasma 25-hydroxyvitamin D (25(OH)D). Female Wistar rats (n 126) received 37.5 micrograms cholecalciferol/d for 14 d and were subsequently studied for a further 88 d. Two subgroups of eighteen rats each were fasted for 3 d immediately after treatment (days 14-17) and at the end of the study (days 98-101). During treatment, plasma cholecalciferol increased rapidly to reach a steady-state. Plasma 25(OH)D and adipose tissue cholecalciferol increased linearly for 1-2 d after treatment. Serum Ca and inorganic phosphate also increased. Subsequently half-lives of plasma cholecalciferol and 25(OH)D, and perirenal and subcutaneous adipose tissue were: 1.4, 22.5, 97.5 and 80.9 d respectively. Fasting, as compared with ad libitum feeding, caused increased plasma free fatty acids, weight loss up to 14% and increased adipose tissue cholecalciferol (nmol/g wet weight). It did not affect plasma cholecalciferol immediately after cholecalciferol treatment, but raised plasma 25(OH)D. Fasting at the end of the study decreased plasma cholecalciferol and increased plasma 25(OH)D. We conclude that orally-administered cholecalciferol rapidly accumulates in adipose tissue and that it is very slowly released while there is energy balance. Fasting causes preferential loss of triacylglycerols from adipose tissue, as opposed to cholecalciferol, but nevertheless augments plasma 25(OH)D. Adipose tissue may act as a 'buffer to functional vitamin D status' by preventing, to a certain extent, unregulated production of 25(OH)D from dietary vitamin D, and by slowly releasing vitamin D under fasting conditions.