We recently directly examined the relationship between blood-oxygen-level-dependent (BOLD) functional magnetic resonance imaging (fMRI) signals and neural activity by simultaneously acquiring electrophysiological and fMRI data from monkeys in a 4.7-T vertical scanner (Logothetis, NK, Pauls J, Augath MA, Trinath T, Oeltermann A. Neurophysiological investigation of the basis of the fMRI signal. Nature 2001;412:150-157). Acquisition of electrical signals in the microvolt range required extensive development of new recording hardware, including electrodes, microdrives, signal conditioning and interference compensation devices. Here, we provide a detailed description of the interference compensation system that can be used to record field and action potentials intracortically within a high-field scanner. (c) 2007 Elsevier Inc. All rights reserved