One of the most influential papers in muscle physiology was published by W. O. Fenn (J. Physiol. London 58: 175-203) in 1923. Fenn determined the quantitative relationship between muscle energy liberation and work performance. Despite the importance of this work, the implications of Fenn's observations are sometimes misunderstood. In this review article, Fenn's experiments are reexamined and the Fenn effect delineated in light of the viscoelastic model of muscle contraction prevalent in the 1920's. The generality of Fenn's results and conclusions are considered in view of more recent results from skeletal and cardiac muscle. Factors responsible for the considerable deviation from Fenn's original results are discussed. Much of the confusion surrounding the generality of Fenn's observations seems to be attributable to the problem of determining an energetic base line for comparing isotonic and isometric contractions. Mommaerts (Physiol. Rev. 49: 427-508, 1969) has suggested the use of an equivalent force base line, which appears to be useful and unifying concept.