Two pathways have been suggested for the conversion of carotenoids to vitamin A in mammals, central cleavage and excentric cleavage. An enzyme, beta-carotenoid-15,15'-dioxygenase (EC 1.13.11.21), has been partly purified from the intestines of several species and has been identified in several other organs and species. The enzyme, which converts beta-carotene into two molecules of retinal in good yield, requires molecular oxygen and is inhibited by sulfhydryl-binding and iron-binding reagents. Most provitamin A carotenoids, including the beta-apo-carotenals, are cleaved to retinal by this enzyme. Its maximal activity in the rabbit is approximately 200 times that required to meet nutritional needs but is less than 50% of that expected to produce signs of vitamin A toxicity. Excentric cleavage unquestionably occurs in plants and some microorganisms and might occur in mammals. Thus far, however, carotenoid dioxygenase with excentric bond specificity has been identified in mammals, the yield of beta-apo-carotenals from beta-carotene in vivo and in vitro is very low, and beta-apo-carotenals are formed nonbiologically from beta-carotene. To remain viable as an alternative pathway of vitamin A formation from carotenoids in mammals, the excentric cleavage hypothesis clearly requires unambiguous direct supporting evidence.