Melatonin is extensively used as a circadian marker rhythm and thus any factors influencing its concentrations other than endogenous rhythmicity must be assessed. We report here the effects of posture on melatonin concentrations in plasma and saliva. The study was performed during the rising phase of the melatonin rhythm between 21:00 and 01:30 h. From 23:00 h, 7 healthy subjects remained sitting until 24:00 h and in dim light (< 10 lux) until 01:30 h. From 24:00 h, they assumed a different postural position in one of the following stages: Stage 1a: 24:00-00:30 h-standing, 00:30-01:00 h-supine, 01:00-01:30 h-standing; Stage 1b-reverse of Stage 1a; Stage 2a: 24:00-01:30 h-supine; and Stage 2b: 24:00-01:30 h-standing. Blood and saliva samples were obtained every hour from 21:00-24:00 h and then every 10 min from 24:00-01:30 h. Plasma and salivary melatonin concentrations, measured by radioimmunoassay, increased when moving from a supine to a standing position and decreased when these positions were reversed. These changes, as seen with other blood components, can be explained through the influence of gravity which causes a decrease in plasma volume on standing and an increase in plasma volume on lying down.