The oral administration of an aqueous extract of 2.5 g of the dried leaves of Solanum malacoxylon (DLSM) produced a rapid hyperphosphataemia, which becomes apparent 4 to 8 h after treatment, in the rabbit. This effect was not accompanied by any significant change in plasma calcium, mangesium, total protein, albumin, urea and creatinine concentration. The urinary excretion of calcium, magnesium, and inorganic phosphate was markedly increased 24 and 48 h after treatment with the extract and was not accompanied by any significant change in the urinary excretion of hydroxyproline. The hyperphosphataemic effect of the DLSM extract was inhibited by Actinomycin D. It appeared that DLSM does not cause bone resorption in normal rabbits maintained on a diet containing adequate levels of calcium and phosphorus. The increased urinary excretion of calcium magnesium and inorganic phosphate after treatment with DLSM extract appeared to be due to increased intestinal absorption. The absence of any significant change in plasma urea and creatinine concentration after treatment with DLSM extract, and the inhibition of the hyperphosphataemic effect by Actinomycin D, indicated that this was a characteristic response of the rabbit to the active principle, and that it was not due to renal damage. The inhibition of the hyperphosphataemic effect of DLSM by Actinomyein D showed that its action involves the transcription of DNA to RNA and protein synthesis.