Radiochemical uptake studies on Ciona intestinalis and Ascidia ceratodes indicated that these animals are able to concentrate vanadium directly from sea water by means of an adsorption mechanism on the mucus sheet. The efficiency of assimilation was of the order of 2.5% for unfed animals. Chemical analyses and radio-autographic studies indicated the vanadium to be localized in the gut region and ovaries. The pumping rate of water by Ciona intestinalis was detd. by a new direct method to be about 2 liters/hour for a mature animal. It was concluded that sea water and its participate constituents could furnish the necessary vanadium demanded by Ciona intestinalis.