Tacrine does not alter the potency of succinylcholine in the rat* Purpose: Tacfine is a cholinesterase inhibitor used to manage Alzheimer's dementia. Given/v, it prolongs succinylcholine blockade in humans but the effects of chronic oral tar.fine are not known. Methods: Groups of adult rats were given 2.5 rng'kg -I tacrine (chronic groups) or I ml saline (control) twice dally by garage for one, two, four or eight weeks. An additional (acute) group r~y..eived 2.5 mg-kg -I tacrine/v. Twelve to 18 hr after the last gavage of tacdne or saline, and -20 rain a~er/v tacrine, cumulative dose-response curves of succinylcholine were determined in the tibialis and soleus muscles in anaesthetized, ventilated rats during monitoring of evoked twitch response to indirect (nerve) train-of-four stimulation. gcsults: The EDs0 and EDgs of sucdnylcholine in control rats were (mean -+ SD) 204 4-41 and 382 -+ 96 /lg.kE 1, respectively, in the tibialis muscle, and 280 -52 and 629 4-168/ug-kg -L in the soleus muscle (P < 0,05 between muscles). In the acute and chronic tacrine groups, the mean EDs0 and EDgs ranged from 166-197 and 277-396 pg.kg -t, respectively, in the tibialis musde, and 248--333 and 546-667 jug.kg -~ , in the soleus muscle. Dose responses did not differ among acute and chronic tacrine groups and the control group.Conclusion: Chronic oral tacdne does not alter muscle response to sucdnytcholine in tt~e rat, This may not apply to Alzheimer patients receiving chronic tacrine since the interaction between acute tacrine and sucdnylcholine in the rat differs from that in humans.