(i) Purpose. The fluid challenge (FC) is a well-established test of preload reserve. Only limited data exist in regard to the FC efficacy based on infusion time. Slow administration may be associated with lack of effect based on fluid redistribution and external conditions changes. On the contrary, fast administration may lead to brisk fluid overload and damage to the endothelium and endothelial glycocalyx (EG). The aim of this trial was to compare the FC infusion time on its hemodynamic effects and EG. (ii) Methods. Prospective randomized single-center trial of fast (5-10 minutes) versus slow (20-30 minutes) administration of 500ml balanced crystalloid FC in spinal surgery (cohort OR) and septic shock (cohort SEP) patients. Hemodynamic response was assessed using standard monitoring and blood flow measurements; damage to EG was assessed using the perfused boundary region (PBR) via intravital microscopy monitoring in the sublingual region within relevant time points ranging up to 120 minutes. (iii) Results. Overall, 66 FCs in 50 surgical and 16 septic patients were assessed. Fluid administration was associated with increase of PBR in general (1.9 (1.8-2.1) vs. 2.0 (1.8-2.2); p= 0.008). These changes were transient in OR cohort whereas they were long-lasting in septic fluid responders. The rate of fluid responsiveness after fast versus slow administration was comparable in global population (15 (47%) vs. 17 (50%); p=0.801) as well as in both cohorts. (iv) Conclusions. Fluid challenge administration was associated with increased PBR (and presumable EG volume changes) which normalized within the next 60 minutes in surgical patients but remained impeded in septic fluid responders. The fluid responsiveness rate after fast and slow FC was comparable, but fast administration tended to induce higher, though transient, response in blood pressure.