By taking up serotonin (5-hydroxytryptamine, 5-HT) released in the extracellular space, the 5-HT transporter (5-HTT) regulates central 5-HT neurotransmission. Possible adaptive changes in 5-HT neurotransmission in knock-out mice that do not express the 5-HT transporter were investigated with special focus on 5-HT1A and 5-HT1B receptors. Specific labelling with radioligands and antibodies, and competitive RT-PCR, showed that 5-HT1A receptor protein and mRNA levels were significantly decreased in the dorsal raphe nucleus (DRN), increased in the hippocampus and unchanged in other forebrain areas of 5-HTT–/– vs. 5-HTT+/+ mice. Such regional differences also concerned 5-HT1B receptors because a decrease in their density was found in the substantia nigra (−30%) but not the globus pallidus of mutant mice. Intermediate changes were noted in 5-HTT+/– mice compared with 5-HTT+/+ and 5-HTT–/– animals. Quantification of [35S]GTP-γ-S binding evoked by potent 5-HT1 receptor agonists confirmed such changes as a decrease in this parameter was noted in the DRN (−66%) and the substantia nigra (−30%) but not other brain areas in 5-HTT–/– vs. 5-HTT+/+ mice. As expected from actions mediated by functional 5-HT1A and 5-HT1B autoreceptors, a decrease in brain 5-HT turnover rate after i.p. administration of ipsapirone (a 5-HT1A agonist), and an increased 5-HT outflow in the substantia nigra upon local application of GR 127935 (a 5-HT1B/1D antagonist) were observed in 5-HTT+/+ mice. Such effects were not detected in 5-HTT–/– mice, further confirming the occurrence of marked alterations of 5-HT1A and 5-HT1B autoreceptors in these animals.