This trial is conducted in Europe. The aim is to compare changes in blood sugar and the number of periods where carbohydrate supplementation is needed to treat low blood sugar, during two different treatments with NN1250 (insulin degludec), a soluble insulin basal analogue (SIBA) in subjects with type 1 diabetes.

Inclusion Criteria:

• Type 1 diabetes mellitus for at least 12 months
• Glycosylated haemoglobin (HbA1c) between 7.5 and 9% (both inclusive)

Exclusion Criteria:

• Donation or loss of more than 500 ml blood or plasma within three months prior to this trial
• Smoker (defined as a subject who is smoking more than 5 cigarettes or the equivalent per day) or not able or willing to refrain from smoking and the use of nicotine gum or transdermal nicotine patches during the in-house periods
• Prior or current treatment with metformin or thiazolidinediones