An electrical connector includes a plug that mates with a receptacle. In a medical application, the plug is connected to electrical leads that pass through a patient's skin to an implanted medical device in the patient's body, while the receptacle is connected to external medical equipment. The plug is small in diameter so the size of the opening in the patient's skin can be minimized. All electrical contacts in the plug are on internal portions. The receptacle includes annular contacts that contact the internal electrical contacts on the plug when the plug and receptacle are properly mated. When the plug is plugged into the receptacle, spring-loaded retention arms in the receptacle lock into place on the plug, retaining the plug in the receptacle.