Dopamine release in the nucleus accumbens (NAc) has been implicated as mediating the rewarding effects of stimulant drugs; however, recent studies suggest that 5-HT release may also contribute. In an effort to assess the role of 5-HT in drug-mediated reward, this study analyzed the serotonergic innervation of NAc using immunocytochemistry for 5-HT and the 5-HT transporter (SERT). We report that in control rats the NAc receives two distinct types of 5-HT axons that differ in regional distribution, morphology, and SERT expression. Most regions of the NAc are innervated by thin 5-HT axons that express SERT, but in the caudal NAc shell nearly all 5-HT axons lack SERT and have large spherical varicosities. Two weeks after methamphetamine or p-chloroamphetamine (PCA) treatment, most 5-HT axons in dorsal striatum and NAc have degenerated; however, the varicose axons in the shell appear intact. These drug-resistant 5-HT axons that lack SERT densely innervate the caudal one-third of the accumbens shell, the same location where dopamine axons are spared after methamphetamine. Moreover, 4 hr after PCA, the varicose axons in the caudal shell retain prominent stores of 5-HT, whereas 5-HT axons in the rest of the NAc are depleted of neurotransmitter. The results demonstrate that two functionally different 5-HT projections innervate separate regions of the NAc and that selective vulnerability to amphetamines may result from differential expression of SERT. We postulate that action potentials conducted from the raphe nuclei can release 5-HT throughout the NAc, whereas transporter-mediated release induced by stimulant drugs is more restricted and unlikely to occur in the caudal NAc shell.