The effects of di-n-propylacetic acid (DPA) on sleep, for some years used as an anticonvulsive drug, has been investigated in 11 healthy volunteers using all-night sleep EEG recordings. DPA acts by an enhancement of the gamma-aminobutyric acid (GABA) level of the brain. Its influence on sleep seemed to be of interest on account of the metabolic relationship of GABA to other short chain fatty acids. After short-term application only a shortening of the time to fall asleep and of the waking time could be found, whereas under long-term administration a decrease in deep synchronous sleep could be observed. In contrast to the results known from animal studies no marked influence on REM sleep was observed. The action of DPA on sleep is similar to that of diphenylhydantoin. No so-called matitudinal ‘hangover’ could be revealed in either d