IT has been reported1–2 that the natural preference of mice for the dark can be reversed by injection of brain extracts taken from donors trained to avoid a black box. This effect was considered to represent a "transfer of learned fear". The possibility remained, however, that the apparent transfer was caused by a substance produced in response to the stress of repeated shocking rather than as a result of learning. A test was therefore carried out to see whether a similar transfer could be obtained with extracts from donors trained to prefer a white box using positive reinforcement.