This trial is conducted in Europe. The aim of the trial is to compare the effect of exercise on blood glucose in subjects with type 1 diabetes, who are treated with either insulin degludec (IDeg) or insulin glargine (IGlar).

Inclusion Criteria:

• Type 1 diabetes mellitus (as diagnosed clinically) for at least 12 months
• Body mass index 18.0-27.0 kg/m^2 (both inclusive)
• Subjects performing regular physical cardiorespiratory activity
• Glycosylated haemoglobin (HbA1c) below or equal to 9.5 %

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 substitute products during the inpatient period
• Supine blood pressure at screening (after resting for at least 5 minutes) outside the range of 90-140 mmHg for systolic or 50-90 mmHg for diastolic