ABSTRACT.Runs of beta activity occur in many healthy subjects during deepening drowsiness. These beta runs are organized in groups with fairly stable lower (around 20/sec) and higher frequency ranges (around 30/sec).Such subvigil beta activities were described first in the 1930ies and 1940ies. These activities are most prominent over the fronto-central region and start with the onset of drowsiness—even at a time when an organized alpha rhythm is still present. They tend to increase in the so-called sleep stage 1 and disappear when spindle activity has become stable in stage 2. Subvigil beta activity reappears during REM sleep yet correlates negatively with rapid eye movements.The stable frequency of the beta runs permits differentiation from EMG artifacts and also from diffuse superimposed beta activity of the hereditary“theta-beta-mixed type.”More difficult is the differentiation of subvigil beta activity front the hereditary patterns of beta groups and certain forms of pharmacogenic frontal beta activities.