Hormonal involvement in the control of blood glucose level ofPoecilobdella viridis was tested by performing debraining, supra- and sub-oesophageal ganglionectomy, followed by injections of the extract of neuroendocrine tissues in operated leeches. Anatomically, the brain ring, through which the oesophagus passes, of this leech is composed of two separate ganglia, the supra-oesophageal and sub-oesophageal, connected to each other by circum-oesophageal connectives. In normal leeches blood glucose level was found to be 10·2 μg%. Removal of complete brain and only the supra-oesophageal portion of the brain produced after 24 h a significant (P<0·01) decrease in the blood glucose concentration. Sub-oesophageal ganglionectomy produced no effect. Injections of brain and supra-oesophageal ganglion extracts into 24 h debrained and supra-oesophageal and sub-oesophageal ganglionectomized leeches increased the blood glucose level significantly, but the injections of sub-oesophageal ganglion homogenates had no effect. These data indicate that supra-oesophageal ganglionic portion of the brain ofPoecilobdella ciridis may be the production site of hyperglycemic hormone.