Input/output profiles of the population responses in CA1 were recorded in the rat hippocampal slice in vitro following stimulation of the Schaffer collaterals before and after theta-frequency patterned primed-bursts (PBs). The most robust potentiation was found for the population spike amplitude, which reached > 200% at low intensity test pulses, but decreased at high intensity. The latency of the population spike was more consistently decreased at high than low stimulus intensities. The enhancement of the population dendritic EPSPs was larger at low than high intensity. Intracellular recordings from CA1 neurons indicated that the intracellular and population EPSPs showed a similar saturation with stimulus intensity, while all single neurons fired at 60 microA intensity when the population spike only reached a mean 70% of its maximal amplitude, suggesting that population spike increase at > 60 microA intensity or following potentiation was caused by increased firing synchrony among neurons. It is suggested that input/output profiles are necessary for the standardization of the degree of long-term potentiation among different laboratories.