1. The hypophysiotrophic area of the rat hypothalamus was studied in vitro. The preparation remained viable for at least 3 hr and showed oxygen consumption varying between 68·9-120 μmole/g.hr. The tissue potassium ion content (per unit wet weight) fell to about 50% of the in vivo concentration during this time compared with 15% in the presence of ouabain (10-4 M). Histological examination of tissue incubated for 3 hr showed variable perineuronal oedema but the nuclei were of normal appearance and none showed the pyknotic changes that would be associated with cell degeneration.