Mixing syringes including a valve mechanism are configured and configurable to enable one or more substances to be prefilled into the syringe. The valve mechanism may be an aspect of a distal seal assembly. Actuation of the valve mechanism permits opening of a fluid passage for the mixing of one or more substances, and the resulting mixed substance can be delivered by the syringe, for example, for administration of a pharmaceutical to a subject. The repetitive mixing of substances contained in a distal and proximal chamber is also provided, where the plunger rod is configured to releasably engage a locking mechanism to actuate the valve, and can axially displace the distal seal assembly while the fluid passage is open.