The first in vivo haemodynamic observations of the recently developed Hall-Kaster central flow prosthetic heart valve are reported. Haemodynamic evaluation of valve function was performed both in acute and chronic experiments following implantation of the Hall-Kaster prosthesis in the mitral position in dogs. These studies showed a low transvalvular pressure gradient, an effective opening angle of about 70 degrees and good diastolic flow through both the large and small orifices of the prosthesis. A slow rotational movement was depicted by the disc during opening and closure.