Objective To evaluate whether hyperglycemia and ageing influence myocardial microvessels and the expression of myocardial PAI-1 mRNA by observing changes in the number of myocardial microvessels and the expression of myocardial PAI-1 mRNA in aged and young diabetic rats. Methods Aged and young SD rats were randomized to diabetes mellitus (DM) group and control group. The DM model was established by intraperitoneal STZ injection. The rats were raised for 8 weeks and sacrificed. Part of the left ventricular tissue was used for immunohistochemical evaluation, and the remaining part was used for detection of PAI-1 mRNA exprerssion. Myocardial microvascular endothelial specific antigen CD34 was detected by using CD34 antibody and EnVision immunohistochemistry. The number of myocardial microvessels was counted by Medical Image Analysis Software. The expression of myocardial PAI-1 mRNA was detected by real-time fluorescence quantitative PCR. Results The number of myocardial microvessels of DM group was significantly smaller than that of the control in all age groups 8 weeks after modeling (P＜0.01), and the number of myocardial microvessels of the aged DM group was significantly smaller than that of the young DM group and young control group (P＜0.01), while there was no significant difference between the aged and young control groups (P＞0.05). The expression of myocardial PAI-1 mRNA in DM rats of all age groups was significantly increased as compared with the control group at corresponding ages (P＜0.01). In addition, the expression of myocardial PAI-1 mRNA in the aged control rats was higher than that in the young control rats (P＜0.05). Conclusion Ageing has a significant influence on the number of myocardial microvessels in diabetic rats but not in non-diabetic rats. Ageing enhances the expression of myocardial AI-1 mRNA in rats. The number of myocardial microvessels is decreased and the expression of myocardial PAI-1 mRNA is enhanced in diabetic rats at all ages.