Cruciate ligaments provide sensory information that cause excitatory as well as inhibitory effects to the activity of the muscles around the knee. The aim of the study was to determine whether these muscular reflexes are reestablished after anterior cruciate ligament (ACL) re-construction. Wire electrodes were inserted during arthroscopy into the normal posterior cruciate ligament (PCL) and the reconstructed ACL in 11 patients who had a successful ACL re-construction 8 months to 12 years earlier. After the anesthesia had subsided, the PCL was stimulated electrically through the electrodes and the sensory threshold was determined. Stimulus amplitudes were increased to 1.5-2.0 times the sensory threshold, and inhibitory reflexes could be elicited from PCL in the quadriceps during active extension and in the hamstrings muscles during active flexion in all patients. Subsequently the ACL re-constructions were stimulated. The sensory threshold was 3.4 times higher in the ACL than in the PCL. Stimulus amplitudes were increased to 1.5-2.0 times the sensory threshold, and a typical inhibitory reflex could be elicited in 9 patients. The latency was the same as for the reflex from the PCL. The stimulus amplitudes necessary to elicit reflexes from the ACL re-constructions were 2, 9 times higher than amplitudes that elicited reflexes from the PCL. Sensation and afferent reflex activity required a much stronger stimulus in the ACL graft compared to normal PCL. We suggest that the reason for this is that the ACL grafts were not reinnervated, and that the reflexes were elicited by spread of stimulus current to the PCL.