Morris studied the Renin–angiotensin system (RAS) for most of his career. His interest in RAS began during his undergraduate studies, when he worked for a while in the laboratory where Eugenie Lumbers had just found early clues to the existence of prorenin (the protein precursor of renin) during her PhD work. He remained interested in the field, and had the good fortune to move to the University of California, San Francisco in the mid-1970s, a center for the development of the tools of biotechnology and molecular cloning. He joined others in applying those tools to RAS, and was among the pioneers is isolating the gene for renin itself, along with the prorenin and kallikrein genes, and the cardiac myosin heavy chain gene.