Noninvasive techniques to monitor reproductive or stress hormones are now widely used in captive and free-ranging wildlife. These methods offer great advantages and deserve to be used also in laboratory rodents. However, we remain naïve about factors that may influence the accuracy of these techniques. The aim of this study was to evaluate the adequacy of measuring the concentration of cortisol fecal metabolites to assess the physiologic stress response. Ten adult female Syrian hamsters were ovariectomized, and all feces voided were collected daily for 4 d before and 5 d after surgery. Cortisol fecal metabolites were extracted and quantified by radioimmunoassay. We determined per-gram fecal cortisol metabolite concentrations, total 24-h fecal output and total 24-h fecal cortisol metabolite production. Surgery considerably affected fecal output, and using per-gram versus total cortisol metabolites led to different conclusions: whereas concentrations increased significantly just after ovariectomy and decreased on subsequent days, the total excreted cortisol metabolites varied in a symmetrical pattern. Therefore, the relative per-gram measure of hormones may not reflect the total amount of circulating hormones, because these measures are comparable only if the volume of the material in which the hormone is contained is the same in the 2 groups.