Background: Peak expiratory flow rate (PEFR) reflects the strength and condition of respiratory muscles and the degree of airflow limitation in large airways. PEFR is recorded in Mini Wrights Peak Flow Meter and it is commonly used for screening the individuals who are asthmatics. PEFR shows hour to hour variation that follows a specific pattern in asthmatics and healthy individuals. Circardian oscillations in the sympathetic activity may have impact on the airway smooth muscles. Materials and Methods: Peak expiratory flow rate was recorded in 60 young healthy females in the age group of 18-25years in Sri Ramachandra Institue of Higher Education and Reasearch. Mini Wright’s peak flow meter was used to measure the peak expiratory flow rate. PEFR was recorded at 6-7am, 9-10am, 1-2pm, 5-6pm, 10-11pm in a single day. Results: Our results showed PEFR values were low in the morning, followed by progressive rise peaking in the afternoon and evening and a small fall at night time. Highest PEFR is seen around afternoon. Conclusion: Comparing the PEFR readings there ia diurnal variation there is a diurnal variation in healthy young females showing a overall dip in the morning PEFR at 6 A.M, peaking in the evening PEFR at 6P.M, eventually decrease in night PEFR at 10P.M. But there is no significant in the afternoon (2P.M) and night (10.P.M).