Adult male rats were implanted with intraventricular (ivt.) brain cannulae for injection of 5 mug of acetylseco-hemicholinium-3 (acetylseco HC-3) as a means of studying acetylcholine (ACh) utilization during morphine withdrawal. Animals were made dependent by implanting s.c. two 75 mg morphine base pellets 24 hrs apart. On the 4th day animals were given 10 mg/kg of naloxone i.p. and/or 5 mug acetylseco HC-3 ivt. and sacrificed by decapitation at various times. The brains were removed and assayed for ACH using a pyrolysis gas chromatographic procedure. Total brain ACh before or after acetylseco-HC-3 was not altered at 5, 30, 60 and 120 but was decreased at 10 min after naloxone. These results are in sharp contrast to our previous data of enhanced brain ACh utilization in withdrawn rats made dependent to morphine by several weeks of twice daily injections. It is apparent that short term morphine pellet administration does not produce the marked neurochemical and behavioral changes of long term morphine injections.