It is well known that the concentration of the thyroid hormone thyroxine increases as day length increases, and the other way around, in Karagouniko and Chios ewes but based on openly international English literature, nothing is known about this hormone on the rams of the breeds mentioned above concerning a meticulous analysis of their aerial environment. So from this perspective, the current research was conducted to investigate the thyroxine concentrations of Karagouniko and Chios rams, taking into account two periods, January-June and July-November, when the day length was increasing and decreasing, respectively. Apart from photoperiod, the meteorological variables air temperature, relative humidity, and sunshine were taken into consideration in the farm of the Artificial Insemination Center of Karditsa (39021΄18΄΄N, 21054΄19΄΄E), Greece (temperate climate zone), where the experimental animals raised. The possible change of thyroxine concentration in both sheep breeds and the possible changes of the abovementioned meteorological variables between the examined periods were detected using t-tests. Higher thyroxine values (P < 0.05) were confirmed in January-June (55.11 ng/ml in Karagouniko and 47.72 ng/ml in Chios rams), in comparison to July-November (49.03 ng/ml in Karagouniko and 44.14 ng/ml in Chios rams). Air temperature and sunshine followed a reverse course (P < 0.05). In both periods, Karagouniko rams were characterized by higher concentrations of thyroxine than Chios rams (P < 0.01). Our results add more insight into the physiology of Karagouniko and Chios sheep related to thyroxine and temperate aerial environment.