Neuronal activity generates ionic flows and thereby both magnetic fields and electric potential differences, i.e. voltages. Voltage measurements at (sub)cellular, meso- and macroscopic level constitute electrophysiology. However, each voltage recording suffers from the isolating and smearing properties of the tissue between source and sensor, is blind to ionic flow direction, and reflects the difference between two electrodes, complicating interpretation, specifically of signal correlations. Magnetic field measurements could overcome these limitations, but have been essentially limited to magnetoencephalography (MEG), using centimeter-sized, helium-cooled extracranial sensors. Here, we report on in-vivo magnetic recordings of neuronal activity in the visual cortex of cats with magnetrodes, specially developed needle-shaped probes carrying micron-sized, non-cooled magnetic sensors based on spin electronics. Event-related magnetic fields inside the neuropil were on the order of several nanoteslas, informing biophysically detailed neural network models, MEG source models and efforts to measure neuronal magnetic fields by other means, e.g. through MRI.