Aim: To assess the effects of evening chronic administration of diazepam on 24-h blood pressure (BP) and heart rate (HR) in healthy young adults. Methods: This randomized double blind, cross-over study evaluated the effects of diazepam 5 mg or placebo, both ingested in the evening, on 24-h ambulatory BP and HR in healthy subjects aged 21–30. Results: A total of 30 subjects were included in the analysis. At the end of 4-week diazepam intake, an increase in 24-h HR mean values was found (+5.2 beats/min, p < 0.05). Analysis of subperiods showed that diazepam produced a 10.1% increase in night-time HR (+6.1 beats/min, p < 0.01) without affecting BP. A significant HR rise (+4.9 beats/min, p < 0.05) and SBP reduction (–3.8 mm Hg, p < 0.05) were observed in the morning hours. The HR increase persisted in day-time hours (+4.6 beats/min, p < 0.05), while BP values resulted unaffected. Conclusions: In healthy subjects, diazepam taken as a hypnotic agent induces a significant HR increase, possibly mediated by a decrease in vagal tone. This effect might be of clinical relevance due to the role that HR plays as an independent cardiovascular risk factor.