Currently, environmental enrichment studies have focused on detection of differences in group means resulting from varied enrichments. The effects of enrichment procedures on variation and hence on study results have received far less attention. Within group variation in studies is directly related to adequate sample size and thus determines ethically correct number of animals to be used. The aim of this study was to evaluate the effects of enrichment items on clinical chemistryand physiological parameters of Wistar rats with the emphasis on the extent of variation. Fortyeight weaned female and male outbred Wistar rats from eight different litters were housed for five weeks in groups of four rats per cage with anaspen tube (20x12x12 em), an aspen block (6x6x6 cm) or without enrichment items (control animals). At the end of the study, adrenals and brown adipose tissues were weighed, growth calculated from initial and final body weights and thefollowing assays were performed from serum samples: AFOS, ALAT, ASAT, LDH, GGT, Pi, Ca, cholesterol, triglycerides, ereatinine, total bilirubin, protein and corticosterone. Based on the variation detected, SOLO Power Analysis was used to calculate the smallest number of animals required (n) to detect an arbitrarily chosen 20 % difference in all means, when significance was set at p=0.05 and statistical power at 0.90. Based on the n-values, additional N-ratio (nenrichment / ncontrol) was calculated to indicate, how many times more or less animals were needed in the enrichment group in comparison to the control group. The enrichment items did not have effects on group means. However. they seemed to influence the variation and hence on the number of animals needed, with enzyme parameters being the most sensitive in this respect. This relationship between enrichment, variation and number of animals needed should be kept in mind while designing experiments with ethically correct sample size
