Γ-valerolactone (GVL), marketed online as “Tranquilli-G” and “excellent Valium”, is used as a legal substitute for γ-hydroxybutyric acid (GHB); however, until now, GVL has only been connected to one Drug-Facilitated Sexual Assault (DFSA) case. Moreover, the pharmaco-toxicological effects of GVL are poorly studied. The aim of this study was to investigate the 1) in vivo effects of gavage administration of GVL (100–3000 mg/kg) on neurological (myoclonia, convulsions), sensorimotor (visual, acoustic, and overall tactile) responses, righting reflex, thermoregulation, motor activity (bar, drag, and accelerod test) and cardiorespiratory changes (heart rate, breath rate, oxygen saturation, and pulse distension) in CD-1 male mice and the 2) in silico ADMET profile of GVL in comparison to GHB and the open active form γ-hydroxyvaleric acid (GHV). The present study demonstrates that GVL inhibits, in a dose-dependent manner, sensorimotor and motor responses and induces cardiorespiratory depression (at a dose of 3000 mg/kg) in mice. The determination of the ED₅₀ in sensorimotor and motor responses revealed that GVL is about 4–5 times less potent than GHB. In silico prediction of ADMET profiles revealed toxicokinetic similarities between GHB and GHV, and differences with GVL. These results suggest that GVL could be used as a substitute for GHB and should be added to forensic toxicology screenings.