Endplate-targeted botulinum toxin injections can achieve optimal neuromuscular blockade. The goal of this study was to identify the motor endplate (MEP) zone in the human psoas muscle through dissection. In 24 human cadaver psoas muscles the nerve branches and their intramuscular course were followed by stereoscopic microscopic dissection as far as their terminal ramifications. From the lumbar plexus, an average of 3.7 (range 2-7) nerve branches enter the psoas muscle. The proximal and distal limit of the MEP-zone are situated at about 30% and 70%, respectively, of the distance between the twelfth thoracic vertebra (Th12) and the passing of the psoas under the inguinal ligament. In reference to the sacral promontory (P), these limits are respectively from 50% of the Th12-P distance to 20% of the P-pubis distance. This study of the MEP zone of the human psoas muscle can allow the clinician to inject BTX-A close to its site of action. Muscle Nerve, 2010.