This article is available from: http://www.jcircadianrhythms.com/content/4/1/6[Background] We have recently reported that spontaneous internal desynchronization between the locomotor activity rhythm and the melatonin rhythm may occur in rats (30% of tested animals) when they are maintained in constant dim red light (LLdim) for 60 days. Previous work has also shown that melatonin plays an important role in the modulation of the circadian rhythms of running wheel activity (Rw) and body temperature (Tb). The aim of the present study was to investigate the effect that desynchronization of the melatonin rhythm may have on the coupling and expression of circadian rhythms in Rw and Tb.[Methods] Rats were maintained in a temperature controlled (23–24°C) ventilated lightproof room under LLdim (red dim light 1 μW/cm2 [5 Lux], lower wavelength cutoff at 640 nm). Animals were individually housed in cages equipped with a running wheel and a magnetic sensor system to detect wheel rotation; Tb was monitored by telemetry. Tb and Rw data were recorded in 5-min bins and saved on disk. For each animal, we determined the mesor and the amplitude of the Rw and Tb rhythm using waveform analysis on 7-day segments of the data. After sixty days of LLdim exposure, blood samples (80–100 μM) were collected every 4 hours over a 24-hrs period from the tail artery, and serum melatonin levels were measured by radioimmunoassay.[Results] Twenty-one animals showed clear circadian rhythms Rw and Tb, whereas one animal was arrhythmic. Rw and Tb rhythms were always strictly associated and we did not observe desynchronization between these two rhythms. Plasma melatonin levels showed marked variations among individuals in the peak levels and in the nightto-day ratio. In six rats, the night-to-day ratio was less than 2, whereas in the rat that showed arrhythmicity in Rw and Tb melatonin levels were high and rhythmic with a large night-to-day ratio. In seven animals, serum melatonin levels peaked during the subjective day (from CT0 to CT8), thus suggesting that in these animals the circadian rhythm of serum melatonin desynchronized from the circadian rhythms of Rw and Tb. No significant correlation was observed between the amplitude (or the levels) of the melatonin profile and the amplitude and mesor of the Rw and Tb rhythms.[Conclusion] Our data indicate that the free-running periods (τ) and the amplitude of Rw and Tb were not different between desynchronized and non-desynchronized rats, thus suggesting that the circadian rhythm of serum melatonin plays a marginal role in the regulation of the Rw and Tb rhythms. The present study also supports the notion that in the rat the circadian rhythms of locomotor activity and body temperature are controlled by a single circadian pacemaker.This work was supported by the NASA Cooperative Agreement NCC 9–58 with the National Space Biomedical Research Institute to G.T.Peer reviewed