beta carotene absorption in the unanesthetized rat was investigated by recirculating a micellar perfusate that contained beta-carotene through jejunal and ileal intestinal loops. Radioautography revealed extensive distribution of the provitamin throughout the layers of the small bowel. A linear relationship was found between the concentration of beta-carotene in the perfusate and its rate of absorption at perfusate concentrations of 0.5--11 mM. Increases in the perfusate hydrogen ion concentrations, additions of fatty acids of varied chain lengths and degrees of saturation, and an increase in the perfusate flow rate caused higher rates of beta-carotene absorption. Increase in the perfusate sodium taurocholate concentration above 2.5 microM did not change the absorption rate of beta-carotene. These experiments indicate that beta-carotene absorption takes place by passive diffusion. The process of diffusion can be modulated by intraluminal factors that change the physical characteristics of perfusate or stimulate the intracellular cleavage of carotene to retinal.