ABSTRACT This is the final report of an experiment of 20 years’ duration, in which we have repeated, in its essentials, the well-known experiment of William McDougall purporting to reveal a Lamarckian inheritance of the effects of training on rats. The test is one involving light discrimination, and McDougall recorded a steady improvement in the rate of learning on a succession of 32 generations; but he omitted to check the results against a properly conducted control. Our experiment confirms McDougall to the extent that we too have obtained long duration trends of improvement in learning-rate (Figs. 2, 3); but we find that the effect is not sustained, and that it is, moreover, shown also by a control experiment, using animals of untrained ancestry. This forbids a Lamarckian interpretation. Statistical analysis of the data indicates that the ‘condition’ of the rat markedly affects its speed of learning, and that progressive changes in learning-rate, over a succession of generations, are in reality correlated with the health of the laboratory colony, which is subject to periods of decline and recovery.