Preclinical studies in rodents suggest that augmentation of serotonin reuptake inhibitors (SSRIs) therapy by the 5-hydroxytryptamine(1A) (5-HT(1A)) receptor agent pindolol might reduce the delay between initiation of treatment and antidepressant response. This hypothesis is based on the ability of pindolol to potentiate the increase in serotonin (5-HT) transmission induced by SSRIs, an effect achieved by blockade of the 5-HT(1A) autoreceptors in the dorsal raphe nuclei (DRN). However, placebo-controlled clinical studies of pindolol augmentation of antidepressant therapy have reported inconsistent results. Here, we evaluated the occupancy of 5-HT(1A) receptors following treatment with controlled release pindolol in nine healthy volunteers with positron-emission tomography (PET). Each subject was studied four times: at baseline (scan 1), following 1 week of oral administration of pindolol CR (7.5 mg/day) at peak level, 4 h after the dose (scan 2), and at 10 h following the dose (scan 3), and following one dose of pindolol CR (30 mg) (at peak level, 4 h) (scan 4). Pindolol occupancy of 5-HT(1A) receptors was evaluated in the DRN and cortical regions as the decrease in binding potential (BP) of the radiolabelled selective 5-HT(1A) antagonist [carbonyl-(11)C]WAY-100635 or [carbonyl-(11)C] N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridyl)cyclohexa necarboxamide abbreviated as [(11)C]WAY-100635. Pindolol dose-dependently decreased [(11)C]WAY-100635 BP. Combining all the regions, occupancy was 20 +/- 8% at scan 2, 14 +/- 8% at scan 3, and 44 +/- 8% at scan 4. The results of this study suggest that at doses used in clinical studies of augmentation of the SSRI effect by pindolol (2.5 mg t.i.d.), the occupancy of 5-HT(1A) receptors is moderate and highly variable between subjects. This factor might explain the variable results obtained in clinical studies. On the other hand, at each dose tested, pindolol occupancy of 5-HT(1A) receptors was higher in the DRN compared to cortical regions, demonstrating a significant in vivo selectivity for DRN 5-HT(1A) autoreceptors relative to cortico-limbic postsynaptic receptors. This selectivity is necessary for the potentiation of 5-HT transmission, and this finding represents an important proof of concept in the development of 5-HT(1A) agents for this application. Early evaluation of new drugs with PET imaging will enable rapid screening of compounds based on DRN selectivity and more appropriate determination of doses for clinical trials.