The in vitro effect of methylmercury (MM) on the enzymatic activities of brain cell specific marker enzymes, choline acetyltransferase (CAT), glutamic acid decarboxylase (GAD), 2',3'-cyclic nucleotide phosphohydrolase (CNP), glutamine synthetase (GS) and enolase was examined. The results demonstrate that at 100 microM MM, GS activity was not affected whereas a small decrease in the activity of both GAD (20%) and enolase (10%) was observed. CNP and CAT activity appeared to be more sensitive toward MM with 100 microM MM producing inhibition of 50% and 30%, respectively. The addition of sulfhydryl protecting reagents such as DTT or sodium thioglycolate can restore the enzyme activities to normal control levels despite prior exposure of the enzymes to MM.