This trial iss conducted in Europe. The aim of this clinical trial is to evaluate the hypoglycaemic response (the response to low blood sugar) to NN1250 (insulin degludec) in subjects with type 1 diabetes.

The trial is designed as a two-period, crossover trial where the trial participant is randomised to one of two treatment periods.

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:

• Use of insulin glargine within 3 months prior to first dosing of investigational product in this trial
• Subject who has donated any blood or plasma in the past month or more than 500 mL within 3 months prior to screening
• Smoker
• Recurrent severe hypoglycaemia (more than 1 severe hypoglycaemic event during the last 12 months)