This trial is conducted in Europe. The aim of the trial is to investigate the pharmacodynamic properties (effect) by comparing two NN1250 (insulin degludec, IDeg) formulations in subjects with type 1 diabetes.

Inclusion Criteria:

• Type 1 diabetes mellitus (as diagnosed clinically) for at least 12 months
• Body mass index 18.0-28.0 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)
• Not able or willing to refrain from smoking and use of nicotine gum or transdermal nicotine patches during the inpatient period