The ingestion of the leaves of the plant Solanum Malacoxylon (SM) causes widespread soft tissue calcification in the cattle of some areas of Buenos Aires province. The oral or subcutaneous administration of a watery extract of SM leaves induces elevations of serum levels and urinary excretion of calcium and phosphate. To elucidate the mechanism of action of SM upon mineral metabolism, its effect on the intestinal absorption of calcium and phosphate and upon bone resorption rate, was studied in the rabbit. Animals on a low calcium low phosphate diet, received an oral dose of 13 μCi of 45Ca and 25μCi of 32P. Half of the rabbits had received previously 111 mg of SM daily during 5 days. One hour after the administration of the test dose, plasma levels of 45Ca were (Av±lSD): Controls: 2.0±0.6% of doses/100 ml of plasma; SM treated: 3.6 ± 1.1 (p < 0.01). Plasma levels of 32P were: Controls 0.S9 _ 0.4% of doses/100 ml of plasma; SM treated 3.5 ± 1. 4 (p < 0.00S). To study the effect of SM upon bone resorption in vivo, a group of rabbits received a dose of 50 μCi of 45Ca and 30 days later urine was collected during a control period and during the administration of 400 mg of SM along five days. SM caused a rapid increase in the urinary excretion of 45Ca and 40Ca, but due to the greater elevation in the excretion of the former, the specific activity of urinary calcium increased from an average basal level of 579 cpm/mg Ca to 767 cpm/mg Ca returning after the treatment to control level. These results indicate that a watery extract of leaves of SM contains a, yet unknown, factor that augments calcium and phosphate absorption from the gut and bone resorption rate in vivo. Simultaneous studies carried out to compare the effects of SM and vitamin D, tend to demonstrate that the delay in the onset of action is shorter and the effect fades faster in SM treated animals. (Endocrinology90: 563, 1972)