Abstract Neurotransmission can be affected by exposure to heavy metals, such as mercury and lead. ATP is a signaling molecule that can be metabolized by a group of enzymes called ecto-nucleotidases. Here we investigated the effects of mercury chloride (HgCl 2) and lead acetate (Pb(CH 3COO) 2) on NTPDase (nucleoside triphosphate diphosphohydrolase) and ecto-5′-nucleotidase activities in zebrafish brain membranes. In vitro exposure to HgCl 2 decreased ATP and ADP hydrolysis in an uncompetitive mechanism and AMP hydrolysis in a non-competitive manner. Pb(CH 3COO) 2 inhibited ATP hydrolysis in an uncompetitive manner, but not ADP and AMP hydrolysis. In vivo exposure of zebrafish to HgCl 2 or Pb(CH 3COO) 2 (20 μg/L, during 24, 96 h and 30 days) caused differential effects on nucleotide hydrolysis. HgCl 2, during 96 h, inhibited the hydrolysis of ATP, ADP and AMP. After 30 days of exposure to HgCl 2, ATP hydrolysis returned to the control levels, ADP hydrolysis was strongly increased and AMP hydrolysis remained inhibited. Exposure to Pb(CH 3COO) 2 during 96 h caused a significant decrease only on ATP hydrolysis. After 30 days, Pb(CH 3COO) 2 promoted the inhibition of ATP, ADP and AMP hydrolysis. Semi-quantitative RT-PCR analysis showed no changes in the expression of NTPDase1 and 5′-nucleotidase, following 30 days of exposure to both metals. This study demonstrated that Hg 2+ and Pb 2+ affect the ecto-nucleotidase activities, an important enzymatic pathway for the control of purinergic signaling.