The load-strain and stress-relaxation behavior of wet rat tail tendon has been examined with respect to the parameters strain, rate of straining, and temperature. It is found that this mechanical behavior is reproducible after resting the tendon for a few minutes after each extension so long as the strain does not exceed about 4 per cent. If this strain is exceeded, the tendon becomes progressively easier to extend but its length still returns to the original value after each extension. Extensions of over 35 per cent can be reached in this way. Temperature has no effect upon the mechanical behavior over the range 0–37°C. Just above this temperature, important changes take place in the mechanical properties of the tendon which may have biological significance. The application of the techniques used here to studies of connective tissue disorders is suggested. Some of the mechanical properties of tendon have been interpreted with a simple model.