Introduction Preclinical data implicate large conductance calcium-activated potassium (BK_Ca) channels in the pathogenesis of headache and migraine, but the exact role of these channels is still unknown. Here, we investigated whether opening of BK_Ca channels would cause headache and vascular effects in healthy volunteers. Methods In a randomized, double-blind, placebo-controlled, cross-over study, 21 healthy volunteers aged 18–39 years were randomly allocated to receive an intravenous infusion of 0.05 mg/min BK_Ca channel opener MaxiPost and placebo on two different days. The primary endpoints were the difference in incidence of headache and the difference in area under the curve (AUC) for headache intensity scores (0–12 hours) and for middle cerebral artery blood flow velocity (V_MCA) (0–2 hours) between MaxiPost and placebo. The secondary endpoints were the differences in area under the curve for superficial temporal artery and radial artery diameter (0–2 hours) between MaxiPost and placebo. Results Twenty participants completed the study. Eighteen participants (90%) developed headache after MaxiPost compared with six (30%) after placebo ( p = 0.0005); the difference of incidence is 60% (95% confidence interval 36–84%). The area under the curve for headache intensity (AUC_{0–12 hours}, p = 0.0003), for mean V_MCA (AUC_{0–2 hours}, p = 0.0001), for superficial temporal artery diameter (AUC_{0–2 hours}, p = 0.003), and for radial artery diameter (AUC_{0–2 hours}, p = 0.03) were significantly larger after MaxiPost compared to placebo. Conclusion MaxiPost caused headache and dilation in extra- and intracerebral arteries. Our findings suggest a possible role of BK_Ca channels in headache pathophysiology in humans. ClinicalTrials.gov, ID: NCT03887325.