Increased areas of anesthesia in the oral cavity have been shown to significantly impair oral function in normal individuals. In patients who undergo oral cavity reconstruction, loss of sensation plays a major role in producing disturbances in postoperative oral function. Free tissue transfer techniques have permitted the problem of sensory loss to be addressed through the use of sensate cutaneous free flaps, in which microneural anastomoses are performed between a sensory nerve supplying the flap and a recipient nerve in the head and neck. To critically assess the results of such reconstructions, the effect of sensory restoration on oral cavity rehabilitation must be studied. As a first step toward this goal, normal values for sensory discrimination of the floor of mouth and tongue are needed. Previous studies of oral sensation failed to examine the ventral tongue and floor of mouth. The purpose of this study is to determine the surface sensibility of these regions in healthy patients and in patients who received radiation therapy to the oral cavity. Sensation was evaluated using static and moving two-point discrimination in 90 healthy subjects divided equally into three age groups: 20 to 40 years, 41 to 60 years, and 61 to 80 years. In addition, 20 patients who received radiation therapy were studied. The mucosa of the dorsal and ventral aspects of the lateral tongue, tongue tip, and floor of mouth was examined. The tongue tip is the most sensitive area, followed by the dorsal lateral tongue, ventral lateral tongue, and floor of mouth. The effects of age and radiation therapy on sensory discrimination are discussed.(ABSTRACT TRUNCATED AT 250 WORDS)