Levels of norepinephrine (NE), dopamine (DA) and the main metabolite of serotonin 5-hydroxyindoleacetic acid (5-HIAA) have been measured in the suprachiasmatic nuclei (SCN), preoptic area (PA), and median eminence (ME) of hypothalamus of rats after sole subcutaneous injection of 1,2-dimethylhydrazine (SDMH). Circadian changes of DA in all the brain structures under study as well as of NE in PA were observed in the control group, their levels in the mornings being higher than in the evenings; a circadian change of 5-HIAA in SCN had an opposite tendency. Both the evening (11 p.m.) and morning (11 a.m.) administrations of SDMH at the dose of 21 mg/kg body weight resulted in disturbances of all the circadian rhythms observed in control. In some cases only a 12 hrs circadian rhythms phase shift was found, in the others these rhythms of neurotransmitters disappeared entirely. The evening administration of SDMH, unlike the morning one, resulted in an increase in total NE content in the hypothalamic structures under study. It is suggested that the effect of SDMH on the levels and circadian rhythms of neurotransmitters in the hypothalamic structures under study is due to affecting activities of the enzymes of biogenic amines synthesis, synaptic transmission, melatonin synthesis and secretion rhythms, as well as to its genotoxic influence upon the genes controlling circadian actions.