ABSTRACT: The sublethal effects of tributyltin oxide (TBT) at concentrations of 2 to 128 ng l^-1 in sea water on imposex development, weight, activity and feeding rate of the adult common dogwhelk Nucella lapillus (L.) were investigated over 12 mounder conditions conforming to the UK Good Laboratory Practice standard. TBT promoted imposex development, with a no-effect concentration of <2 ng l^-1 for exposure of 6 mo or more. The degree of imposex did not exceed levels which causedsterility in any treatment group. Low levels of imposex in controls showed seasonal changes in intensity. Seasonal patterns were overridden by 2 ng TBT l^-1 or more. The low numbers of mortalities observed were not related to TBT concentration.The activity of dogwhelks (tendency to climb the tank walls) varied seasonally, reflecting natural behaviour patterns of wild populations. TBT exposure had no effect on this activity. The feeding rate (predation on mussels Mytilus edulis) showedmarked seasonal variations in all tanks, but there was no correlation with TBT exposure. Exposure to TBT had no effect on the tissue weight of male dogwhelks, but the body weight of females was negatively correlated with TBT after 12 mo exposure. Meanbioconcentration factors (water to tissue of dogwhelks) ranged from 7400 to 25000, and decreased with increasing TBT exposure concentration. It is estimated that the feed mussels accounted for approximately 40% of the TBT accumulated by the dogwhelks.