1. Sub-oesophageal ganglia taken from Periplaneta americana L. which had been kept in reversed conditions of light and darkness have been implanted into the abdomens of cockroaches living in normal light: dark conditions. When implantation was continued for 4 days transplantable tumours appeared by the eighteenth day in the midgut of the majority of cockroaches. 2. No tumours appeared in the guts of cockroaches when the implanted suboesophageal ganglia had been taken from animals kept in normal conditions of light and darkness, nor when brains or corpora allata were implanted. 3. Injection of an extract of five sub-oesophageal ganglia in 0.05 ml. Ringer solution produced tumours only when injection took place during the inactive phase of the 24 hr. locomotory rhythm. 4. The implantation of sub-oesophageal ganglia taken from animals kept in normal light:dark conditions into animals kept in reversed light:dark conditions was followed by tumour formation only if the implantation was carried out daily for 8 clays. The incidence of tumour formation is lower than under the conditions described in 1. 5. Details are given of the histology of the developing tumours. 6. When tumour formation had begun the implantation of sub-oesophageal ganglia with secretory phases in phase with that of the animal's own ganglion prevented further growth of the tumour. The cells of the tumour had, however, undergone an irreversible change for, as soon as implantation ceased, growth began once more.