We previously reported that treatment of neonatal rats with 3,4-methylenedioxymethamphetamine (MDMA) led to decreased serotonin transporter (SERT) binding in the neocortex and hippocampus at postnatal day (PD) 60 (Meyer and Ali, SFN, 2001). The present study used immunohistochemistry for the serotonin transporter (SERT) with quantitative image analysis to investigate the long-term consequences of neonatal MDMA exposure for the serotonergic system. Male rats (n=12/group) were given s.c. injections of 10 mg/kg (±)-MDMA or saline vehicle b.i.d. from PD 1 - PD 4. At 9 months of age, the animals were anesthetized and perfused transcardially, 40-μm parasagittal sections were cut, and the sections were processed for SERT immunohistochemistry (Vector Elite kit) followed by silver-gold intensification of the diaminobenzidine reaction product. Gray-level thresholding was used to assess the percent area covered by SERT-immunoreactive fibers in 10 different forebrain regions. All measurements were made by a rater blin...