It has been previously demonstrated that gabapentin, a gamma-amino butyric acid analogue, inhibits monoaminergic neurotransmitter release from rabbit caudate nucleus slices and from rat cortex. In humans this drug has been shown to have anti-epileptogenic activity. Serotonin may act as an inhibitory neurotransmitter and its interaction with blood platelets is thought to reflect its central actions. We investigated sleep stages, whole blood serotonin levels, and serum melatonin in healthy men after the administration of gabapentin. With increasing serum gabapentin levels six healthy subjects showed an increase in sleep stages 3 and 4 and in whole blood serotonin (P less than 0.05) Serum melatonin levels were not influenced. On account of these results we speculate that gabapentin modulates the release of serotonin from blood platelets. The increase in peripheral serotonin points paradigmatically to an increase in the bioavailability of serotonin which may account for the increase in sleep stages 3 and 4.