1. Intravascular pressures and angiographic records have been made on the giant earthworm, Glossoscolex giganteus. The measurements were made on unanaesthetized animals after previous implantation of indwelling catheters.

2. The pressures developed by the peristaltic activity of the dorsal vessel showed an average systolic value of 24 cm. H2O while the diastolic value was 14 cm. H2O. The frequency was variable around 6-8 contractions per min.

3. Angiographic analysis showed the peristaltic activity to include about 25 segments and to move at a velocity of 12 segments per second at 18° C. The lumen of the dorsal vessel was almost fully constricted during peristalsis.

4. The peristaltic activity of the dorsal vessel was responsible for the filling of the lateral hearts. These in turn showed an increased force of contraction at each peristaltic contraction.

5. The five pairs of lateral hearts usually contracted in synchrony. Their frequency of contraction was considerably higher than the peristalsis of the dorsal vessel (20 beats per min.).

6. The pressures developed in the ventral vessel in response to the lateral heart contractions were considerable and exceeded 100 cm. H2O during vigorous activity. All intravascular pressures were markedly increased by general motor activity of the earthworm.

This work was supported in part by grant GB-358 of the National Science Foundation, by grant HE-02557 of the National Heart Institute and by a travel grant of the Fulbright Commission to the Junior author.