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During an investigation into certain aspects of the carotene metabolism of ruminants, it was desired to obtain an indication of the effect of the composition of the feed on the availability of carotene to the animals. The most convenient method of measuring approximately the carotene absorption appeared to be by determining its apparent digestibility using the ordinary digestibility-trial technique. However, if any appreciable decomposition occurred in the digestive tract, the calculated digestibility might bear no relationship to the actual absorption. It seemed desirable, therefore, to verify whether there is any oxidative decomposition in regions where absorption of carotene does not occur by determining carotene content, relative to an inert reference substance, at various points through the digestive tract. In,addition, it was anticipated that the same experiment would afford some explanation of an apparently anomalous excretion of carotene observed in preliminary digestibility trials, where some of the sheep were found to be excreting more carotene than they were consuming, excretion in some cases reaching 160 % of the ingested provitamin. EXPERIMENTALLignin as rejmence substance. Lignin was selected as a suitable reference substance for these experiments, and a digestibility trial with pasture-fed sheep provided an opportunity for checking the recovery of lignin from grass. At the same time, the excretion of carotene was further investigated by estimating carotene : lignin ratios in samples of faeces collected at intervals from these animals.Estimation of carotene and lignin. Carotene was estimated in feed, ingesta and faeces by a modification (McGillivray, 1950) of the cold-extraction method using a 'foaming mixture' of light petroleum and ethanol as described by Moore & Ely (1941). The method of Ellis, Matrone & Maynard (1946) was used for the lignin determinations.All assays were carried out in duplicate and, in order to reduce sampling errors, the residue from the carotene determinations was dried at room temperature, ground, and used for the lignin estimations. Carotene:lignin ratios were calculated in all instances in mg. carotene/g. lignin. The reproducibility of these ratios was investigated in a number of instances, a typical result, where six determinations were made on a well-

SummaryIt has been established that the conversion of carotene to vitamin A in the sheep occurs in the wall of the intestine. This conclusion is based on surviving tissue experiments in which sections of intestine have been incubated with carotene and the product conclusively identified as vitamin A by colorimetric and spectrophotometric methods. It is further supported by the high vitamin A levels in intestinal as compared with non-intestinal lymph, and similar observations with cattle suggest the intestine as the site of conversion in this species also.

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