We have used an in vitro model system of the circadian clock, dispersed chick pineal cells, to examine the effects of temperature on the circadian clock of a homeotherm. This preparation enabled us to isolate a circadian clock from in vivo homeostatic temperature regulation and expose cells to both constant temperatures and abrupt temperature changes. By manipulating the temperature of the pineal cells, we have demonstrated that (1) the circadian clock compensates its period for temperature changes over the range of 34–40 degrees C; Q10 = 0.83, a value within the range of Q10 values measured for poikilothermic circadian clocks; (2) temperature pulses (42 degrees C, 6 hr duration) shift the phase (advance and delay) of the circadian rhythm in a phase- dependent manner; and (3) a temperature cycle (18 hr at 37 degrees C, 6 hr at 42 degrees C) will entrain the circadian clock in vitro. This is the first demonstration of temperature entrainment of the circadian clock of a homeotherm in vitro. In addition we have found that temperature directly influences the synthesis and release of melatonin, the primary hormonal product of the pineal gland. The biosynthesis of melatonin is strongly temperature dependent with a Q10 > 11 when melatonin release is measured at ambient temperatures between 31 degrees C and 40 degrees C. In contrast, 6 hr 42 degrees C temperatures pulses acutely inhibit melatonin release in a manner similar to that seen previously with light pulses. These results demonstrate that a circadian clock from a homeothermic vertebrate is temperature compensated, yet temperature cycles can entrain the circadian melatonin rhythm. Thus, the chick pineal circadian oscillator has retained all the fundamental properties of circadian rhythms.