A previous finding (Bindra, Williams, & Wise, 1965) that Ss respond "different" more quickly than "same" when comparing the pitch of two tones was confirmed. It was found that this effect could be enhanced by making same trials less frequent and that the effect could be reversed by making same trials more frequent. The experimental treatment also affected the error rates of the two responses. Error and latency results are discussed in terms of a model of R T based upon a sequential-sampling and decision procedure, and are shown to be compatible with such a model. A decision bias towards the response "different" must be taken into account when the model is applied. This bias appears to be influenced by the size of the interstimulus interval used.