Measurements have been made on the stress-strain relationship for the atomic lattice of iron (purity 99·95%). The changes in dimensions of the atomic lattice of tensile specimens have been determined in the direction perpendicular to the applied stress and compared with the contraction in external dimensions which occurs in the same direction. It has been shown that the lattice stress-strain curve, obtained by plotting the lateral change in lattice spacing against the tensile stress, exhibits three main characteristics. Up to the external yield point, the lattice contraction is directly proportional to the applied stress. Beyond the yield point, the lattice contraction slows down as the stress increases. Finally, at still higher stresses, the lattice tends to expand. A further set of experiments, in which cycles of stress are employed, show that when a stress greater than the yield stress is applied and then removed from a specimen, the lattice is left with a permanent expansion which depends in a regular manner upon the value of the stress applied. It is concluded that beyond the external yield point, the iron lattice undergoes two distinct modifications. First, the lattice spacing tends to contract elastically, in conformity with the external elastic contraction exhibited by the specimen in the same direction (perpendicular to the applied stress). Second, the lattice tends to deform in a manner which leads to a superposed expansion of lattice spacing. The actual change of spacing at a particular stress is then the resultant of the two effects; a process which explains the unexpected shape of the lattice stress-strain curve, and affords systematic information on the problem of internal strains in metals. Further experiments indicate that the permanent expansion of the lattice after loading occurs not only in the direction perpendicular to the applied stress, but also in the direction of the stress; the permanent lattice deformation thus represents a decrease in density of the test specimen. Finally, the results of some experiments are recorded on the effect of temperature on the lattice expansion effect, and it is shown that recovery of the lattice can be produced by mild heat treatment at a temperature much lower than that required to renew the properties of the metal by recrystallization.