Long-term potentiation (LTP) was induced by unilateral tetanic stimulation of the perforant path in the dentate gyrus of rats with implanted electrodes. Evoked potentials were monitored for the subsequent 3 days in one group, and for 23 days in another. The dentate gyrus was removed bilaterally and slices prepared and stored in 10% DMSO/Krebs for subsequent analysis of K+-stimulated, Ca2+-dependent release of [3H]glutamate. In the 3-day group, in which the mean increase in the population EPSP was 35% at the time of sacrifice, release from the tetanized side was significantly greater than from the unstimulated side. In the 23-day group, both the increase in the EPSP and the increase in release from the tetanized side had declined to statistically insignificant levels. These results extend to a period of several days the previously observed association between LTP and increased K+-stimulated, Ca2+-dependent release of transmitter.