Notable examples of the combined use of optogenetics an fMRI (i.e. “opto-fMRI”) have been recently published, revealing the possibility to map the brainwide substrates modulated by focal neuronal population. However, opto-fMRI is complicated by the use of invasive cranial implants, and the need to control the insidious contribution of heat-induced hemodynamic–responses. Here we show that “chemo-fMRI”, e.g. the combined us of DREADD-based chemogenetics and fMRI, permits to overcome these limitations, by enabling non invasive brainwide mapping of tonically-stimulated neuromodulatory systems. Chemo-fMRI mapping of serotonin-producing neurons is described as an illustrative example of the power of this novel investigational approach.