Healthy Control Sample (n = 70): Sixty of these volunteers will have brain PET scans and 10 of these have only blood sampling to compare (R)-[(11)C]rolipram levels in artery and vein. Healthy subjects (ages 18-55) will be selected who have not met criteria for any major psychiatric disorder, have no known first-degree relatives with mood disorders, and have a current score on the Hamilton Depression Rating Scale (HDRS; 17 item) (Williams 1988) in the not depressed range (less than or equal to 7). Control subjects will be matched to depressed subjects for age and gender. Forty of these subjects will have (R)-[(11)C]rolipram PET scans with blood sampling and 10 subjects have only blood sampling without PET scan. The healthy volunteers who are used to measure the difference in (R)-[(11)C]rolipram concentration between the artery and the vein will not undergo psychiatric assessment because the data will not be compared with those of patients.