We measured interaural time difference (ITD) sensitivity of 72 cells in the inferior colliculus of the anaesthetised guinea pig as a function of frequency and interaural level difference (ILD). For many units there was a "null" frequency, where varying the ILD made no difference to the position of the peak of the ITD sensitivity. This null frequency was not necessarily at the characteristic frequency (CF): it occurred at CF in less than a third of the neurons for which we had sufficient data (14/50). Equally often, the null occurred below (15/50) and less often, above CF (8/50). The remaining (13/50) neurons showed clear phase changes, but these were erratic or parallel and no null could be attributed. In 33 of the 37 neurons with an identifiable null frequency, the peak ITD moved towards the recording side with increasing ILD, for frequencies above the null, and away for frequencies below the null. The changes in ITD sensitivity expressed as phase were maximally about 0.2-0.3 cycles. Many of the changes in response phase with ILD are in the same direction and magnitude as changes in the phase locking with sound level in auditory nerve fibres. Thus, these changes in phase sensitivity at the basilar membrane and auditory nerve are maintained through to ITD tuning in the IC. This is consistent with a coincidence detection mechanism. However, some of the more complex phenomena which we observe are consistent with convergence at the IC.