A 21 week experiment was conducted with male SPF Ico/Shoe: WIST rats to study the influence on diagnostic parameters of toxicological studies of (i) acidification of drinking water by hydrochloric acid (untreated tap water vs. pH 3 vs. pH 2), (ii) individual vs. group caging (5 animals/cage), and (iii) ad libitum vs. 10 ml restrictive water supply. Acidification to pH 2 resulted in a slightly but significantly reduced excretion of phenol red, lowered proteinuria and a decreased urine volume, whereas all other parameters remained unchanged. Individual caging was less stressful than expected from published data. Red blood cell counts were increased, water consumption and urine volume were somewhat lowered, but stress-sensitive parameters like adrenal weight, leucocyte and lymphocyte counts were not altered. A 10 ml restrictive water supply decreased urine volume, food consumption, body weight development and organ weights. Furthermore transient increases in red blood cell counts and hemoglobin contents, leucopenia and--most important--an impaired renal function were observed. In conclusion acidification of drinking water with hydrochloric acid should not be lower than pH 3, male Ico/Shoe: WIST rats can be regarded as minimum susceptible to individual caging, and reduced water intake might give false positive nephrotoxic effects.