Magnus and Starling reported that pituitary extracts caused expansion of the kidney and a marked and often prolonged diuresis. This was the first indication that the neurohypophysis plays a part in the regulation of urine secretion.

A classic work on muscle tone and posture, a subject upon which Magnus spent many years of study. He demonstrated among other things that the labyrinth is the one sense organ entirely concerned with posture and equilibrium. In this book Magnus also described the reflexes involved in mammal posture. The Magnus & De Kleijn reflexes are named after Magnus and his colleague Adriaan de Kleijn (1883–1949). The head and neck reflexes of mammals cause the body to follow automatically when the head moves. He also researched the reflexes of the intestines and phenomena such as motion sickness. English translation, New Delhi, 1987.

Magnus demonstrated the function of the otoliths and semicircular canals of the inner ear in regulating the equilibrium of the body.