The restricted environmental stimulation technique or REST is a method of relaxation where the level of environmental sensory inputs is kept very low. A particular REST technique called tank flotation, or flotation REST, consists of 1 h sessions in a tank containing water with a high salt content and maintained at 35.5 degrees C. In this protocol, five normal subjects were studied before and during 2 h after a 60 min flotation REST session and a control session of 60 min in a supine position on a bed. Cortisol, thyreostimulating hormone (TSH), thyroxine (T4), prolactin, melatonin, luteinizing hormone (LH), growth hormone (GH), beta-endorphin, vasopressin (ADH), gamma-aminobutyric acid (GABA) and homovanillic acid (HVA) were measured in plasma. HVA, 5-hydroxy-indoleacetic acid (5-HIAA) and vanylmandelic acid (VMA) were measured in urine. There were no changes in hormones concentrations that could be attributed to flotation REST. The urinary excretion of VMA was lower after the flotation REST session. The psychological consequences of flotation REST were more easily demonstrated than the neuroendocrine changes that are assumed to reflect the state of relaxation. Flotation REST increased subjective levels of sedation and euphoria. The possible mechanisms by which flotation REST induces relaxation are discussed.