This is a study of the process of interaction between the responses of muscle spindles to stimulation of two fusimotor fibres. Combined stimulation of a static and a dynamic fusimotor fibre supplying the same muscle spindle in the soleus muscle of the anaesthetised cat gave a response which was larger than from stimulating each fibre separately, but less than their sum. A similar summation process was observed with pairs of static fusimotor fibres. The mean summation coefficient for the responses to stimulation of 14 pairs of static fusimotor fibres was 0.29 (range 0.14-0.52; S.D. 0.09), while for 42 static:dynamic pairs it was 0.30 (range 0.07-0.89; S.D. 0.20). Mechanisms considered for the summation process were probabilistic mixing of impulse traffic from two or more impulse generators within the terminals of the primary ending of the spindle, the spread of generator current from one encoding site to another and mechanical interactions between contracting intrafusal fibres. In an experiment where single static and dynamic fusimotor fibres were stimulated together, and then stimulation of the static fibre stopped, the size of the continuing dynamic response was larger than when the dynamic fibre had been stimulated alone. This finding suggested some kind of mechanical interaction between the contracting intrafusal fibres and implies that static and dynamic fusimotor effects within a spindle cannot be considered to be entirely independent of one another.