We have described a simple technique for the measurement of picomoles or nanomoles of ammonium. Samples can be easily processed on the day of an experiment. analysis of 14C-glutamate can be done at a later date by an accurate sensitive and reliable method. We can readily process 50 samples per day. The sensitivity can be increased by using smaller volumes of ammonia-free solutions, prereacting endogenous ammonia with nonradioactive 2-oxoglutarate, and increasing the specific activity of 2-oxoglutarate. The volumes must, however, be very low so as not to diminish the ammonium concentration and thereby limit glutamate formation.