The endplate (+EP) and non-endplate (-EP) distribution of molecular forms of acetylcholinesterase (AChE) was compared in the dimorphic levator ani and diaphragm muscles from adult male rats. Enzyme activity was measured by the thiocholine method and AChE forms were separated on the basis of solubility in sodium phosphate buffer of different ionic strength. For the dimorphic levator ani muscle, total AChE activity was 324.6 +/- 18.9 nmol ASCh hydrolyzed min-1 muscle-1, 90% of which was globular and predominated in the -EP region (78%). The asymmetric forms were almost exclusively detected in the +EP region (9%). In diaphragm muscle, total AChE activity was 176.7 +/- 11.0 units; 66% was mainly globular and located in the -EP region (56%); the asymmetric forms (34%) were either in -EP (11%) or +EP (23%) regions. Thus, a greater proportion of globular form was present in the dimorphic levator ani muscle than in diaphragm muscle. In view of the control exerted by testosterone on dimorphic muscles, it is suggested that the greater synthesis of the globular form in the levator ani occurs under the trophic influence of testosterone.