A 5-day study, forming the initial part of two 32-day clinical trials and involving 22 unipolar depressed inpatients, was designed to determine the relationship between response to sleep deprivation and 24-h urinary MHPG (3-methoxy-4-hydroxyphenylethylene glycol) levels. A statistically significant, positive relationship was noted between the sleep deprivation, modified total HAM-D (Hamilton Depression Rating Scale) scores and MHPG levels at postsleep deprivation, indicating that the greater the severity of depression during sleep deprivation the higher were the MHPG levels after the sleep deprivation period.