Figure 2.Diagrammatic representation of the control of production and the functions of melatonin, with regard to seasonal and circadian timing mechanisms. RHT- retino-hypothalamic tract, NA ” norepinephrine (noradrenalin), SCN – suprachiasmatic nucleus, PVN – paraventricular nucleus, SCG – superior cervical ganglion. The melatonin rhythm is generated by a closed loop negative feedback of clock gene expression in the SCN, Clock and Bmal, positive stimulatory elements, Per, Cry, negative elements, CCG, clock controlled genes. The SCN via neural pathways drives the pineal melatonin rhythm. Per and NAT mRNA oscillate in the pineal although post transcription control is evident in some species. Melatonin influences SCN activity via one or more receptors. Melatonin conveys photoperiodic information influencing the pattern of per expression in the pars tuberalis for the control of seasonal prolactin variations. Melatonin target sites in the hypothalamus influencing seasonal variations in reproductive hormones have yet to be fully defined, although the premammillary hypothalamus is implicated (34). Based on an original diagram by Dr Elisabeth Maywood, MRC Laboratory of Molecular Biology, Neurobiology Division, Hills Road Cambridge, CB2 2QH, UK.