The controversial ‘memory transfer’ experiments developed during the last few years into a new bioassay approach for the study of the chemical correlates of learning and the molecular coding of neural information. As with all biological assays, the reliability of the method is limited because of the multiplicity of the factors determining success or failure. The experience accumulated in the last five years in many laboratories explains most of the past failures and suggests conditions for the best assay designs. With the help of this method, we were able to identify a pentadecapeptide, called scotophobin, formed in rat brain during acquisition of dark avoidance. It is hoped that a sufficient number of these coded molecules will be identified in the near future to give us a broader base for studying the molecular mechanism of neural information processing.