Vessel for transdermal diffusion cell testing includes a container defining an interior chamber having an opening against which skin is placed, and a casing arranged partially around and spaced apart from part of the container to define a compartment therebetween. The chamber retains a saline solution and is not in flow communication with the compartment through which water is circulated. The vessel includes separate inlet ports and outlet ports, each including a conduit communicating with the chamber or compartment. The outlet port of the chamber is arranged above the inlet port of the chamber, proximate the opening of the chamber, and is angled downward relative to a horizontal upper surface of the container against which skin being tested is placed. Tilting of the vessel prior to or during introduction of solution prevents air bubbles from remaining between the skin and the solution.