The article explores the hypothesis that the suprachiasmatic nucleus (SCN) of the hypothalamus functions as an autonomous circadian pacemaker. Using a Halasz knife, researchers isolated a hypothalamic "island" containing the SCN in rats and examined its rhythmicity. The study found that when the SCN was isolated, rhythmicity persisted within the island but was lost in other brain regions, suggesting the SCN is an independent oscillator. In blinded animals, rhythmicity was maintained in the island but not in other brain areas, indicating the SCN is not reliant on external inputs. The SCN's rhythmicity was not affected by the absence of the eyes, suggesting the eye is not the pacemaker. The study also demonstrated that the SCN's rhythmicity could persist in constant darkness and under LD cycles, further supporting its autonomous function. The rhythmicity within the SCN was distinct from other brain regions, with a daytime maximum, and was not influenced by hormonal factors. The results strongly suggest that the SCN is the primary circadian pacemaker in mammals, operating independently of other brain areas. The study provides strong evidence for the SCN's role as an autonomous circadian oscillator, with implications for understanding circadian rhythms in mammals.The article explores the hypothesis that the suprachiasmatic nucleus (SCN) of the hypothalamus functions as an autonomous circadian pacemaker. Using a Halasz knife, researchers isolated a hypothalamic "island" containing the SCN in rats and examined its rhythmicity. The study found that when the SCN was isolated, rhythmicity persisted within the island but was lost in other brain regions, suggesting the SCN is an independent oscillator. In blinded animals, rhythmicity was maintained in the island but not in other brain areas, indicating the SCN is not reliant on external inputs. The SCN's rhythmicity was not affected by the absence of the eyes, suggesting the eye is not the pacemaker. The study also demonstrated that the SCN's rhythmicity could persist in constant darkness and under LD cycles, further supporting its autonomous function. The rhythmicity within the SCN was distinct from other brain regions, with a daytime maximum, and was not influenced by hormonal factors. The results strongly suggest that the SCN is the primary circadian pacemaker in mammals, operating independently of other brain areas. The study provides strong evidence for the SCN's role as an autonomous circadian oscillator, with implications for understanding circadian rhythms in mammals.