Growth hormone (GH) and prolactin (PRL) secretion were evaluated in 28 patients who had sleep apnea or narcolepsy but no other primary neurologic or endocrine disorders. Eighty-one percent of subjects with impaired alertness failed to demonstrate serum GH concentrations in excess of 5 ng per milliliter following oral administration of L-DOPA, 500 mg. Diminished GH responses to sleep and intravenous arginine were observed in 57 percent and 44 percent, respectively, of patients tested. Sleep-related PRL release was less than normal in women with narcolepsy, with or without sleep apnea. All patients had at least one abnormality in GH or PRL secretion.