The effects on the choroid plexus (CP) of chronic (22 weeks) exposure to 1, 10 or 100 ppm Cd2+ in the drinking water was studied in adult mice. The CP from these mice was examined using scanning and transmission electron microscopy. The mice receiving 10 or 100 ppm Cd2+ showed a dose dependent reduction in fluid intake and weight loss. The CP from animals receiving 100 ppm Cd2+ showed severe degenerative changes characterized by loss of microvilli, rupturing of the apical surface, increased cytoplasmic vacuolation and cellular debris. There was also an increase in cellular blebbing and increased dark/light cell ratio. Cerebrospinal fluid analysis revealed an increased protein content. The mice receiving 10 ppm Cd2+ showed similar but less severe degenerative changes. The 1 ppm Cd2+ group were indistinguishable from controls. These results suggest that it would be appropriate to examine CP function in individuals excessively exposed to Cd.