1. The influence of potassium ions upon the lipolytic hormone effect in vitro was examined on epididymal white as well as interscapular brown adipose tissue slices.
2. In either adipose tissue, the omission of potassium from incubation medium markedly, though not completely, diminished the catecholamine effect on lipolysis.
3. Similar inhibition was obtained by the addition of ouabain. This inhibitory effect of ouabain was well overcome in white adipose tissue by increasing the potassium concentration in the medium, whereas in brown adipose tissue no interaction between ouabain and potassium was observed.
4. Lipolytic action of theophylline was not affected at all by ouabain in both adipose tissues and was significantly increased in the absence of potassium in the medium.
5. Catecholamine-stimulated increase in the respiration of brown adipose tissue was almost completely abolished by ouabain or potassium omission.
6. It is suggested that two mechanisms may be involved in the enhanced lipolysis. It is possible that potassium ions play some essential role in the operation of these mechanims.