This trial is conducted in Europe. The aim of this trial is to compare pharmacodynamic (the effect of the investigated drug on the body) and pharmacokinetic (the exposure of the trial drug in the body) properties of insulin degludec and insulin glargine 300 U/mL at steady-state conditions in subjects with type 1 diabetes mellitus.

Inclusion Criteria:

• Male or female aged 18-64 years (both inclusive) at the time of signing informed consent
• Subjects diagnosed (clinically) with type 1 diabetes mellitus at least 365 days prior to the day of screening
• Body mass index 18.5-29.0 kg/m^2 (both inclusive)

Exclusion Criteria:

• Subject who has donated any blood or plasma in the past month or more than 500 mL within 3 months prior to screening
• Smoker (defined as a subject who is smoking more than 5 cigarettes or the equivalent per day)who is not able or willing to refrain from smoking or use of nicotine gum or transdermal nicotine patches during the in-patient period