Biliary transport maximum (Tm) and relative storage capacity (S) for sulfobromphthalein (BSP) were compared in male and female Fischer-344 rats of different ages ranging from 3 to 30 mth. When expressed per unit liver weight, the male Tm value decreased rapidly with age reaching 40% of the 3-mth-old value at 30 mth. Female values, which were significantly lower than corresponding male values during the first year also decreased steadily with age but the rate of decrease was much lower than in male values. Consequently, at 30 mth there was no longer any difference between male and female rats. S values expressed per gram liver stayed essentially unchanged with age in both male and female rats, although female values were always lower than male values. The liver's in vitro conjugation capacity for BSP with glutathione also decreased significantly with age in male rats, while in female rats it remained unchanged between 3 and 28 mth of age. Thus, the decrease in BSP Tm value with age could not be fully explained by the decrease in liver's conjugation capacity with glutathione. The results of the present study, coupled with our previous observations on other rat strains suggest that, at least in rats, BSP Tm decreases with age but S does not, despite contrary reports made by other authors on man and rats.