The mobility of 5-doxylstearic acid spin label (5-SASL) in the intact rat liver Golgi membranes of streptozotocin diabetes was studied as a function of free blood sugar level and temperature. During development of diabetes, indicated by the increase of the free blood sugar level, the membrane fluidity measured in the physiological temperature range (1) does not change in comparison with control in light diabetes, (2) decreases significantly in advanced diabetes and (3) again increases to the control level in heavy diabetes (the free blood sugar levels being 200-250 mg/100 ml, 250-350 mg/100 ml and greater than 350 mg/100 ml, respectively). The development of streptozotocin diabetes is accompanied by significant changes in lipid composition of liver Golgi membranes as also shown in our previous observations. The measurements of motion of 5-SASL in Golgi membranes as well as in vesicles, made from commercially available lipids of composition close to the liver Golgi membranes, show that a decrease of cholesterol contents is the main factor which induces the increase membrane fluidity. We suggest that in the heavy diabetes the hemostatic regulation in the lipid composition leads to minimization of alterations in membrane fluidity to obtain comparatively normal activity of certain membrane enzymes.