CLASSICAL, imprinting of precocial birds has been studied in the laboratory for some 20 years. Suggestions have also been made over a similar period about the imprinting of precocial mammals, but no systematic experiments specifically concerned with imprinting have been reported so far. Although Shipley's study of guinea-pigs1 referred to imprinting, in reality it was concerned with the approaches and following responses of these animals to moving objects. Imprinting involves more than that, namely an attachment to a given figure, and this can be readily assessed in a discrimination test2. The experiment reported here describes imprinting in young guinea-pigs, judged in terms of the animals' preference for familiar, compared with strange, objects.