1. A one-trial passive avoidance learning task using day-old chickens is described and shown to be suitable for testing the effects of direct intracranial injection of drugs. 2. Inhibitors of the sodium pump have been found to block the formation of long-term memory only if present at the time of learning. 3. Inhibitors of protein synthesis will prevent long-term memory formation even if they are administered after learning. 4. From this, there appear to be at least two stages in the formation of memory. A short-term phase dependent on membrane mechanisms leads to permanent storage dependent on protein synthesis.