"Equipotential" doses of lormetazepam, triazolam and flunitrazepam were tested regarding their influence on K-complexes, sleep spindles and rapid eye movements. The receptor affinity was used to determine the equipotency. In this respect triazolam, lormetazepam and flunitrazepam show a relationship of 1:2:4. Lormetazepam, however, showed only slight changes of the three neurophysiological parameters even when given as double dosage. Triazolam and flunitrazepam reduce K-complexes considerably, increase sleep spindles enormously and reduce REM activity markedly; both substances suppress the first REM period, flunitrazepam even the second one. All three of the benzodiazepines have the same effect on the dissociation of K-complexes and sleep spindles; epochs with K-complexes or sleep spindles are favored compared to epochs with K-complexes and sleep spindles. The general reduction of K-complexes and the increase in sleep spindles can be interpreted as a sleep protective function. When deciding on the dosage, however, REM-suppression should be used as a guide since it has the strongest effect on the cyclic structure of sleep which nowadays is considered to be the safest evidence of well balanced sleep behavior.