Urethane is a standard anesthetic utilized for in vivo recordings in the hippocampus. In studies of long-term potentiation (LTP), the measure of interest is the response amplitude minutes to hours following train delivery. In the absence of experimental treatment, we have consistently observed upward drift in the amplitude of the population spike (PS) and EPSP slope of the dentate gyrus (DG) evoked field response in acute surgical preparations performed in the urethanized rat. The present study systematically monitored PS amplitude and EPSP slope in the DG every 30 minutes for 6 hours following optimal positioning of Teflon-coated bipolar stainless steel electrodes under urethane anesthesia. At maximal stimulus intensities, large time-dependent increases in PS amplitude (70-80%) were observed over the first 2-4 hours, an effect that was exaggerated at lower stimulus intensities. Increases in the EPSP slope were smaller in magnitude (20-30%) and stabilized within a shorter period of time (1-2 hours). Animals were warmed on a heating pad and body and brain temperature remained constant over the recording session. Reducing stimulating electrode size and recording with glass micropipettes did not alleviate the upward drift in response amplitude. Similar increases were also seen under pentobarbital anesthesia. To dissociate anesthetic from surgical effects, recordings were obtained from animals previously prepared with indwelling electrodes and injected with urethane. Although slight declines (10-15%) in EPSP slope occurred over time, no significant alterations in PS amplitude were seen in the chronic preparation at high stimulus intensities. Low stimulus intensities yielded a more variable response pattern and, in direct contrast to the acute preparation, time-dependent declines, not increases, were noted in both parameters. These data suggest that generalized surgical trauma contributes to the upward drift in response amplitude and indicate that long stabilization periods are required in acute surgical preparations for accurate field potential recordings.