In vivo measurement techniques provide a direct, noninvasive method for determining human body composition. Normal aging, alterations resulting from disease and dysfunction, and the efficacy of therapy protocols can be readily assessed on the basis of body composition studies. Accurate measurement of the patient's skeletal mass, muscle mass, protein mass, total body water, intracellular and extracellular fluid, and body fat can be made. Data obtained by in vivo neutron activation techniques developed at Brookhaven National Laboratory, whole body gamma spectrometry and computer analysis yield the desired information on body composition. The complete system is referred to as in vivo neutron activation analysis (IVNAA). This proposal seeks support for updating the detection system by incorporating significant technological changes in detection equipment that improve both the resolution and the efficiency of the system. Such improvement will increase the quality of the measurements made and bring benefits to the patients. Central to the modernization is the improvement of the detection system. At present, the detection array consists of two banks of NaI(T1) cylindrical detectors, one above and the other below the bed on which the patient rests during the measurement process. Each bank consists of nine rows of three detectors, for a total of twenty-seven detectors per bank, or 54 detectors for the two banks. It is proposed to replace these fifty-four detectors with thirty-two square (4"""""""" x 4"""""""" x16"""""""") NaI(T1) crystals. Their shape ensures an improvement in the geometry. Additionally, it is proposed to replace the nineteen-year old interface unit with one that embodies up-to-date efficient electronic components. The present system is difficult to keep in good repair, as some of the parts requiring replacement have become obsolete. Only the original multi-channel analyzer/computer system (built, as was the interface system in 1965) was replaced in FY83 to provide modernization of the electronics. Significant improvement in this area was achieved. The proposed updating of the remainder of the system should reestablish the facility to the """"""""state-of-the-art"""""""" status it enjoyed when first developed. These modifications will significantly enhance the precision and the accuracy of body composition measurements obtained to implement a number of NIH-funded clinical research programs.