A mathematical model was used to analyze the contributions of different types of ionic currents in the pyramidal cells of the longitudinal CA3 slice to the magnetic fields and field potentials from this preparation. Murakami et al. (2002) showed that a model based on the work of Traub (Traub et al. 1991, 1992, 1993) provides a quantitatively accurate account of basic features of three types of empirical data - magnetic fields outside the slice, extracellular field potentials within the slice and intracellular potentials within the pyramidal neurons - elicited by stimulations of the soma and apical dendrites. Here this model was used to compute the net current dipole moment (Q) due to each of the different voltage- and ligand-gated synaptic channels in the cells in presence of fast (A-type) g- aminobutyric acid (GABAA) inhibition. These Q's are proportional to the magnetic field and electrical potential far away from the slice. The intrinsic conductances were found to be more important than synaptic conductances in determining the shape and magnitude of the Q's. Among the intrinsic conductances, the sodium (gNa) and delayed-rectifier potassium (gK(DR) channels were found to produce sharp spikes. The high- threshold calcium channel (gCa) and C-type potassium channel (gK(C)) primarily determined the overall current waveforms. The roles of gCa and gK(C) were independent of small perturbations in these channel densities in the apical and basal dendrites. A combination of gNa, gCa, and gK(C) accounted for most of the evoked responses except for later slow components which were primarily due to synaptic channels.