Salt and water balance and vasopressin secretion were measured in three colonies of Sprague-Dawley rats. Although sodium and water retention were similar between the groups, there were marked differences in both the rate and diurnal pattern of intake and excretion. Animals housed under semi-barrier conditions showed a lower basal plasma vasopressin concentration but were more sensitive to physiological stimuli. However, since pathogenic status and environmental conditions cannot entirely explain these results, genetic variation is likely to be a contributory factor. © 1993, Royal Society of Medicine Press. All rights reserved.