Most studies of the effects of inhalation anesthetics on somatosensory evoked potentials (SSEPs) have examined SSEP at single times after initiation of an anesthetic. This study describes SSEP changes as functions of time of exposure to isoflurane. Both transient and sustained SSEP changes were observed. Nonlinear regression was used to fit exponential terms to the trend curves for end-tidal anesthetic concentration and SSEP peak latency. End-tidal concentrations could be well described by two exponential terms, one with a long and one with a short time constant. Isoflurane at 1% inspired concentration produced a sustained SSEP latency change that could be fit by a single exponential term; the time constant was essentially identical to the long time constant of end-tidal concentration. The long time constants ranged from 7 to 33 min. At 0.5% isoflurane, SSEP changes were often small or not sustained, and the changes could not always be well described by an exponential curve. These data suggest that the time-course of anesthetic effects on SSEPs may be prolonged and complex, and the possibility of changes over time should be considered both in experimental studies and during intra-operative monitoring.