Diurnal variations in pineal serotonin, norepinephrine, and epinephrine levels were measured at four different time intervals (0600, 1200, 1800, and 2400 hr) in adult male softshelled turtles. The serotonin level increased during day and decreased during night. But the norepinephrine and epinephrine levels showed reverse changes. The findings reveal that these is a distinct circadian rhythm in serotonin, norepinephrine, and epinephrine contents of the pineal-paraphyseal complex in adult male soft-shelled turtles. © 1986.