The present investigation was to determine the extent to which mercury (Hg) provokes measurable effects on the structure of the digestive gland of slugs as well as to relate the extent of these effects to the cell and tissue distribution of Hg. For this purpose, slugs (Arion ater) received various dietary concentrations of Hg (from 0 to 1,000 microg Hg/g food) as chloride for 30 days and were histologically examined every third day. Autometallography was used to demonstrate Hg as black silver deposits (BSD) in paraffin sections. The lysosomes and residual bodies of digestive cells resulted to be the major accumulation sites. In addition, Hg was also evidenced in lipofuscine granules of vacuoles in excretory cells but, however, it was rarely observed within calcium cells. Generally, the extent of BSD increased with dietary Hg concentration and exposure time but, however, it became significant lowered after exposure to 1,000 microg Hg/g food for 30 when the digestive epithelium appeared almost devoid of digestive cells. On the other hand, significant changes were recorded in the quantitative structure of digestive tubules. Mean Epithelial Thickness (MET), Mean Luminal Radius (MLR) and Mean Diverticular Radius (MDR) were recorded as measures of the sublethal biological effect of Hg. MET, MLR/MET and MET/MDR were affected by Hg concentration (C), exposure time (T) and CxT interaction, changes in MET, MLR, MLR/MET and MET/MDR being explained by regression models after logarithmic transformation of the data. In order to explain the nature of the changes in the quantitative structure of the digestive tubules this investigation was complemented with qualitative histological observations. According to them, the excretory activity in digestive cells was initially enhanced. Afterwards, the relative numbers of digestive cells declined until the extreme cases of exposure to 1, 000 microg Hg/g for 27 to 30 days in which the digestive epithelium was mostly comprised of calcium and excretory cells. Concomitantly, some changes took place in blood vessels where Leydig cells became disrupted and the connective tissue layers thickened. Finally, it is suggested to use slugs in soil quality assessment as sentinel organisms ("Slug Watch") in which biomarkers of exposure to metallic pollutants and of biological effect are recorded.