Objectives: This study aims to investigate the effect of the hot pack application on the morphology of healthy median and ulnar nerves. Patients and methods: Between August 2021 and September 2022, a total of 54 healthy volunteers (17 males, 37 females; mean age: 31.9±9.4 years; range, 21 to 63 years) were included in the study. The cross-sectional area (CSA) and depth of the right median and ulnar nerve were measured using ultrasonography before and after 20-min hot pack application. The right median nerve CSA and depth from the level of the scaphoid bone (at the level of the carpal tunnel) and from the mid-forearm were also measured. The right ulnar nerve CSA and depth from the level of the hook of hamate and the mid-forearm were evaluated. The depth measurements between the skin and the outer hyperechoic border of the nerve were performed. Results: After the hot pack application, there was an increase in both the median nerve CSA at the carpal tunnel (from 0.06±0.01 to 0.09±0.02 cm2) and forearm (from 0.06±0.02 to 0.09±0.02 cm2) levels (p<0.001) and the ulnar nerve CSA at the hook of hamate (from 0.04±0.03 to 0.06±0.01 cm2) and forearm (0.05±0.01 to 0.08±0.02 cm2) levels (p<0.001). No statistically significant difference was observed between the pre- and post-application depth measurements (p>0.05). Conclusion: Our study results show that the hot pack application leads to acute swelling of the median and ulnar nerves. In future studies, the relationship between morphological changes in healthy and pathological nerves and electrodiagnostic findings should be investigated.