The accumulation of beta-carotene in serum and skin was evaluated in human volunteers. A single 51-mg dose of beta-carotene given in the absence of dietary fat resulted in no detectable change in serum beta-carotene. The same dose administered with 200 g fat increased serum beta-carotene 2.5-fold at 40 h. Similarly, administering beta-carotene daily in three divided doses with meals raised the serum beta-carotene concentration three times as high compared with the same total dose administered once a day; both regimens had the same time constant for serum accumulation; 9-10 d. Remittance measurements of skin color demonstrated that the accumulation of beta-carotene in skin was delayed by up to 2 wk compared with serum accumulation. These data indicate that beta-carotene absorption requires dietary fat and is enhanced by administering with meals but there is a long time constant for serum (10 d) and tissue (several weeks) accumulation.