Short segments of hippocampal EEG in the rat were recorded during rapid-eye-movement sleep (REM) and active waking behaviors. Fourier analysis was used to reveal the time-varying changes of theta power, frequency, coherence and phase in stratum radiatum (apical dendritic layer) of CA1 region. In all cases studied during REM sleep or active waking, a significant, positive correlation of radiatum theta phase and peak theta frequency was found. The radiatum phase was found to range from 100 to 170 degrees and increased towards 180 degrees at high (7.8-8.6 c/sec) theta frequency. Electrodes at sites other than stratum radiatum did not show this phase-frequency relation. By selectively averaging across spectra of EEG with the same selected peak theta frequency, it was found that the power at the first or second theta harmonic, the integrated power of fast EEG (30-100 c/sec) and the radiatum theta phase all generally increased with increasing theta frequency. The results indicate that the radiatum theta phase (and frequency) is dynamically varying during normal behaviors. Together with previous findings on the pharmacology of theta phase, the results suggest that both atropine-sensitive and atropine-resistant inputs are present during normal behaviors.