The therapeutic effect of insulin alone or insulin combined with 30 min delayed thrombolytic therapy was investigated in rats embolized in the right hemisphere with a fibrin clot made from autologous blood. Animals were killed seven days after embolization and the infarct volumes were measured in % of the affected hemisphere. Mortality was calculated as the number of animals dying spontaneously before the scheduled euthanasia. The median infarct volume in control animals (n = 12) was 24%. Insulin (3 IU kg(-1)) was given subcutaneously 15 min, 3 h, and 24 h after embolization (n = 12) and reduced median infarct volume to 11%. Human recombinant tissue plasminogen activator, 8 mg kg(-1), was infused intravenously during 45 min starting 30 min after embolization (n = 14), and the median infarct volume was 18% in this group. When the two treatments were combined, the median infarct volume was reduced to 11% (n = 14). The infarct volumes in the treatment groups were not significantly different from controls (p = 0.62, Kruskal Wallis test). Mortality rates increased from 0% among controls to 47% (p = 0.01) in the insulin alone and 38% (p = 0.02) in the combination therapy group. In conclusion, insulin treatment aiming at blood glucose levels around 2-4 mmol l(-1) was detrimental to clinical outcome causing significantly increased mortality.