This paper is a further contribution to the study of the mode of action of the amphibian heart and gives an account of radiographic studies on the hearts of the frog and toad, Rana temporaria and Bufo bufo in which a radio-opaque medium (thorotrast) injected through the left pulmonary vein into the left atrium has been followed in its course through the heart by means of rapid serial radiographs (approximately 3 per second).

The results show that although blood from the left atrium (represented by the thorotrast) will, at ventricular diastole, come to occupy the left side of the ventricle at systole it is propelled towards the base of the conus where it mingles to a very great extent with the blood from other parts of the ventricle. The radiographs also show that the thorotrast obtains access to both sides of the conus and also to all three arterial roots on both sides.

The evidence brought forward supports the earlier results obtained by similar injections of indian ink made by Vandervael (1933) and by Foxon (1948).

A brief review of the problem of the mode of action of the heart of the frog is given and it is shown that most recent authors favour the view that there is little, if any, selective distribution of the blood in frogs and toads.