For the first time pineal glands obtained from 5 male rats flown aboard an orbiting satellite (Soviet Biosatellite #1887) were analyzed for their melatonin, serotonin (5-HT), 5-hydroxyindole acetic acid (5-HIAA), and calcium content. In addition, plasma 5-HT and 5-HIAA were measured. These parameters were compared to indicators of gonadal function: plasma testosterone concentration and spermatogonia development. Plasma melatonin was low at the time of euthanasia (lights on) and was not different among the experimental groups (flight animals, synchronous controls, and vivarium controls). Pineal calcium of flight animals was not different from ground controls. However, pineal 5-HT and 5-HIAA in the flight group were significantly higher than those in ground controls (p less than 0.05). These findings suggest a possible increase in pineal 5-HT turnover in flight animals which may result in increased melatonin secretion. Since melatonin is known to possess antigonadal properties, the alteration of pineal 5-HT turnover and its expected effects on melatonin secretion may, in part, explain the lower plasma testosterone levels (p less than 0.001) and 4-11% fewer spermatogonia cells (p less than 0.02) observed in flight animals.