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Abstract: Assuming that temporal fluctuations in physiological parameters (e.g. haematology, biochemistry) in individual healthy non-exposed animals are non-adverse, the minimal magnitude of the Critical Effect Size (CES) for a number of continuous parameters of toxicity studies was derived. A total of 36 studies (19 pharmaceutical preclinical studies in dogs and 17 chemical risk assessment studies in rats) were analysed to determine within-animal variation in their control groups. Minimal CES-values were derived for each group of studies, differentiating where necessary between strains and sexes, using the 2.5 percentile (lower limit) and/or 97.5 percentile (upper limit) of the distribution of the within-animal variation around the mean of each parameter. We concluded that minimal CES-values for continuous clinical chemistry and haematology parameters should be established separately per species, strain, sex and study duration investigated. Grouping of minimal CES-values, leading to more or less “general” values, seems possible for those parameters that are subject to tight homeostatic control and consequently show little within-animal variation. Nearly a quarter of the proposed CES-values is ⩽5%, nearly a quarter range from 6% to 10%, a quarter is 15% or 20%, and nearly 30% of the proposed values is ⩾20% of the mean of the control animals. [Copyright &y& Elsevier]