Extracts of the calcinogenic plants Solanum malocoxylon and Cestrum diurnum stimulate phosphate absorption by the jejunum of vitamin D-deficient chicks, as determined by everted gut sac technique. Their action on cellular pathways of transepithelial phosphate transport is indistinguishable thereby from that of cholecalciferol. Increased net absorption from the lumen was due to enhanced uptake of phosphate from the luminal side, while leakage of tissue phosphate in the opposite direction was apparently unaffected. Steep serosa/mucosa concentration gradients were observed as consequence of enhanced levels of transepithelial phosphate flux in the mucosa-to-serosa direction. With respect to their stimulatory action on phosphate absorption, the calcinogenic plant factors retained their biological activity when phosphate transport was depressed by a high strontium diet. Their action in overcoming the strontium inhibition of phosphate absorption, calcium-binding protein synthesis, and alkaline phosphatase activity, was comparable to the effect of 1,25-dihydroxycholecalciferol. On the basis of these biological responses, the action of the plant factors from Solanum malacoxylon and Cestrum diurnum provides further evidence for their close resemblance to the hormonally active sterol.