A two-generation bioassay on sodium saccharin (NaS), involving 2500 second-generation male rats, was designed to determine the dose response for urinary bladder tumours in male rats and to evaluate other changes possibly related to the occurrence of the tumours. Six treatment groups (125-700 rats/group) were fed dietary levels of NaS ranging from 1.0 to 7.5%. To evaluate the role of in utero exposure, two additional groups were exposed to NaS either only during gestation via dams fed diet containing 5.0% NaS or for a single generation beginning at birth. In the latter group, the nursing dams were placed on an NaS diet immediately after giving birth and their offspring were weaned onto diets containing 5.0% NaS. A third additional group, included to evaluate the specificity of NaS and the role of excess sodium in the occurrence of urinary bladder tumours, was fed diet containing sodium hippurate (NaH) for two generations--5.0% NaH to the first generation and to the second until 8 wk old, and subsequently 3.0% because of unexpected toxicity. A clear dose response for urinary bladder tumours was observed in the second-generation NaS-treated male rats. The steep slope of the dose-response curve indicated a rapid decline in tumour incidence with decreasing dose. The 1.0% dietary level (fed to 700 rats) was considered to be a no-effect level for bladder tumours. The only other treatment-related pathological changes were an increase in urinary bladder weight in rats fed greater than or equal to 3.0% and an increase in mineralization of the kidneys with greater than or equal to 1.0%. Several physiological effects were seen in the NaS-treated groups showing an increase in bladder tumours (i.e. those fed greater than or equal to 3.0%). Some changes, e.g. depressed growth and increased water consumption, were indicative of a general disturbance of these rats, but analysis of body-weight, food-consumption, compound-consumption and water-consumption data revealed no correlations within any dose group between these quantitative data and the occurrence of bladder tumours. Other changes indicative of the compromised situations of the rats fed high dietary levels of NaS were anaemia in weanling rats fed 5.0 or 7.5% and a reduction in litter size at dietary levels greater than or equal to 3.0%. Changes in urine volume and urine osmolality were highly correlated with the occurrence of the urinary bladder tumours.(ABSTRACT TRUNCATED AT 400 WORDS)