In the present study, two hundreds and forty healthy albino young (n=120) and old (n=120) rats were used during winter and summer season. Rats were divided into four groups in each season i.e. young and old, consisting of male (n=30) and female (n=30) in each age category. In each age ´ sex matched rats, three subgroups were made and have been given the name as control group (CC), horizontal restrained group (HR) and head down suspended group (HDS). For winter season, the room temperature of experimental period ranged from 20 to 23°C and for summer season, the experimental room temperature ranged from 30 to 33°C. A 12 hours light/12 hours dark cycle with ad libitum food offered each day to an individual rats as well as fresh water (at normal temperature) were provided every day from 9-10 h (morning Rats were decapitated on day 7^th (n=5) and day 28^th (n=5) of experimental period from all groups to collected the blood in a hepranized tubes for the estimation of lymphocytes and neutrophils. Appropriate statistical analysis was performed to estimate the difference between age, days, treatments and their possible interactions during each season. During winter and summer seasons, male and female rats did show a significant decrease in percent of lymphocytes, however a significant increase in the neutrophils percent was also observed in the HR and HDS groups. During summer, a significant increase in neutrophils and a decrease in lymphocytes were observed in male and female rats of HR and HDS groups.

Key words: Lymphocyte, Neutrophil, Rats, Weightlessness