This paper presents the implementation and evaluation of a minimally invasive surgical system whereby a surgeon can maintain as natural a spatio-motor mapping as possible using a 4 DOF constrained slave robot. One way to achieve this interface is to interpose a robotic system between surgeon and patient which will act as a translator between the 6 degree-of-freedom open surgery-like motions of the surgeon and corresponding endoscopic motions within the patient. Our goal was to find the interface that was easiest to learn. We investigated the effects of different master-slave mappings (screen-mapped versus instrument-mapped) and master dexterities (6DOF versus 4 DOF) by means of performance measures noted on simple surgical tasks. This interface has been found to allow motions very similar to those used in natural prehension and open surgery. The particular combination of instrument-based master-slave mapping and 4DOF master has been found to be especially advantageous.