Body sodium (Na), calcium (Ca), and chloride (Cl) of guinea pigs weighing between 227 and 600 g were measured by total body neutron activation analysis (TBNAA) followed by chemical analysis (CA) on 12-17 animals. Paired t test was used to compare any differences in the results obtained by the two methods. There was no significant difference in the results for the three elements. The means and standard deviation for Ca (g/100 g body wt) 1.070 +/- 0.132 (TBNAA) and 1.107 +/- 0.125 (CA); for Na 0.149 +/- 0.019 (TBNAA) and 0.143 +/- 0.021 (CA); and for Cl 0.126 +/- 0.009 (TBNAA) and 0.132 +/- 0.024 (CA). Neutron activation analysis alone, in a series of 27 animals, gave means (g/100 g body wt) and standard deviation of 1.110 +/- 0.084 for Ca, 0.120 +/- 0.009 for Cl, and 0.153 +/- 0.011 for Na. TBNAA has potential usefulness, particularly in longitudinal studies in the same animal, because of its accuracy and the rapidity and ease with which the measurements can be made.