A method has been described for the study of tissue sulfate-conjugating systems in vitro. Liver slices from embryonic chicks were maintained in vitro in a medium containing labeled inorganic sulfate and phenol. It was found that more of the sulfate was esterified at 20 °C. than at 37 °C. due to the longer continued activity at the lower temperature. All sulfate-esterifying activity was lost in liver slices maintained at 37 °C. for 30 hr. while those cultures maintained at 20 °C. continued to esterify sulfate for 70 hr. On the basis of our data there would appear to be a change in the thermal stability of the sulfate-esterifying enzyme system of the chick liver upon its transition from the embryonic stage to the stage of the fully developed chick. Data were presented for the chick 4 months ex ovo. We have been unable to detect any analogous temperature effects upon the sulfate-esterifying system in the livers of embryonic and adult rats.