15 subjects (mean age: 48.2 yr; 13 males, 2 females) with sleep apnea (12 obstructive, 3 central) were treated with an average dose of 2500 mg L-tryptophan (L-T) at bedtime. Comparison of pre- and post-drug polysomnograms showed significant improvement in obstructive sleep apnea but not with central sleep apnea. Most dramatic improvement is seen in subjects with obstructive sleep apnea in non-REM sleep only, but severity of apnea appears to be the most important factor determining improvement. L-T increased REM time and shortened REM latency but had no other significant effects on sleep architecture. Serotoninergic activity with a defect in feedback control of tryptophan-serotonin metabolism is postulated as a potential mechanism in the pathophysiology of obstructive sleep apnea. The enhanced usefulness of L-T in combination with protriptyline is predicted based on early preliminary work at the OSU Sleep Center. The Potential influence of dietary intake on respiratory automaticity is reviewed.