Several investigations have revealed that electrical activity within the central nervous system (CNS) can be affected by exposure to weak extremely-low-frequency (ELF) magnetic fields. Many of these studies have implicated CNS structures exhibiting endogenous oscillation and synchrony as optimal sites for field coupling. A particularly well characterized structure in this regard is the rat hippocampus. Under urethane anesthesia, synchronous bursting among hippocampal pyramidal neurons produces a large-amplitude quasi-sinusoidal field potential oscillation, termed "rhythmic slow activity" (RSA) or "theta." Using this in vivo model, we investigated the effect of exposure to an externally applied sinusoidal magnetic field (16.0 Hz; 28.9 microT(rms)) on RSA. During a 60-min exposure interval, the probability of RSA decaying to a less coherent mode of oscillation, termed "large irregular-amplitude activity" (LIA), was increased significantly. Moreover, this instability persisted for up to 90 min postexposure. These results are consistent with the hypothesis that endogenous CNS oscillators are uniquely susceptible to field-mediated perturbation and suggest that the sensitivity of these networks to such fields may be far greater than had previously been assumed. This sensitivity may reflect nonlinearities inherent to these networks which permit amplification of endogenous fields mediating the initiation and propagation of neuronal synchrony.