Melatonin acts on specific receptors in the suprachiasmatic nuclei (SCN) to phase-dependently regulate the phase of the circadian clock. How the gating of melatonin's effect is restricted to particular times of day is not known, but may be related to temporal differences in receptor availability. In the present study, we used a competitive reverse transcription-polymerase chain reaction (RT-PCR) method to determine if the expression of mt(1) melatonin receptor subtype mRNA in rat SCN varied across the 12:12 light-dark (LD) cycle. Measurement of core body temperature using radiotelemetry confirmed that the male Wistar rats used exhibited a robust diurnal rhythm. mt(1) receptor mRNA was readily detected in reduced SCN slices at all times of day. However, there was no significant variation in the amount of mt(1) mRNA with time of day. Expression of MT(2) melatonin receptor subtype mRNA in reduced SCN slices was confirmed by nested PCR. These results indicate that changes in the level of mt(1) mRNA do not underlie the diurnal and/or circadian variation in the response of the SCN circadian clock to the phase-resetting effects of melatonin.