Abstract: Experiments were performed with the aim of determining the effect of Ascorbic Acid (AA) on diurnal fluctuations in Rectal Temperature (RT) of Bovan Nera pullets during the harmattan season. Thirty experimental pullets were administered AA orally at the dose of 60 mg kg-1, while 20 control birds were given only clean water. Measurements of RT were taken for three days from 07:00 to 18:00 hours during the season using a standard clinical thermometer, inserted through the cloaca into the rectum of each pullet. The overall mean RT of experimental and control pullets were 41.3±0.09°C and 41.3±0.06°C, respectively. The recorded RT in the experimental pullets was lowest at 07:00 hours with the value of 41.2±0.04°C and highest at 15:00 hours with the value of 41.5±0.04°C (p<0.001). In the control pullets, the lowest RT was also recorded at 07:00 hours with the value of 41.0±0.09°C, while the highest value of 41.5±0.07°C (p<0.05) was recorded at 15:00 hours. The RT in both experimental and control pullets rose significantly (p<0.05) and concurrently with the hour of the day (r = 0.668 and 0.457, respectively). The dry-bulb temperature was positively correlated with the RT of both experimental and control pullets (r = 0.991 and 0.948, respectively). The fluctuation in RT of the experimental pullets was 1.1±0.05°C, while that of the control was 1.3±0.08°C (p<0.001). The RT values of both the experimental and control pullets showed distinct diurnal fluctuations. It is concluded that AA administration to pullets ameliorated the thermally stressful effect of the harmattan season and may enhance their productivity and health during the season.