Event-related fields (ERFs) measured by magnetoencephalography (MEG) upon visual stimulation are analysed by Hilbert transformation. The Hilbert transform of real-valued measured ERF is an analytic complex signal, represented by phase and amplitude. The temporal behaviour of the derivative of the phase, i.e. the instantaneous frequency, allows to distinguish time intervals containing meaningful signal from noise. On the basis of both phase and amplitude, energies and latencies of single event ERFs can be determined.