Theta rhythm at the midline of the frontal area can be observed in normal subjects, during mental task performance, rest and sleep. Frontal midline theta rhythm (Fm theta) is a train of rhythmic waves at the frequency of 6- Hz and can be induced by various mental tasks. Fm theta is induced not only during mental tasks but also during nocturnal sleep in which it was most frequent during rapid eye movement (REM) and second most frequent during stage 1 of non-REM (NREM) sleep, and the relationship of Fm theta to dream images during sleep was found. It is concluded, therefore, that the appearance of Fm theta is related to mental activity even during sleep. Fm theta shows individual differences and is related to certain personality traits. High Fm theta groups showed the lowest anxiety score in the Manifest Anxiety Scale (MAS), the highest score in the extrovertive scale of the Maudsley Personality Inventory (MPI) and the lowest score in the neurotic scale of MPI. Low Fm theta groups showed the opposite correlation. Significant negative correlation was found between the amount of Fm theta and platelet monoamine oxidase (MAO) activity. Summarizing the above-mentioned results, it may be concluded that the appearance of Fm 0 is related to mental activity, personality traits and platelet MAO activity. Furthermore, the correlation of such markers as platelet MAO activity and Fm theta with personality traits as measured by various psychological tests may prove to be of great importance in the exploration of the biological bases of personality.