In this paper, the performance of a control law designed for the automatic administration of propofol and of remifentanil in order to track a desired level for the bispectral index (BIS), used as a measure of the depth of anesthesia, is analyzed under the presence of model parameters uncertainties. It is theoretically proved and illustrated by simulations that under these circumstances the controller has a very good performance as the BIS converges to a value contained in a neighborhood of the desired BIS level. A retuning strategy in order to improve the BIS tracking under the presence of uncertainties was also theoretically deduced. Simulations show that this strategy leads to BIS tracking improvement. The performance of the controller in clinical environment is illustrated by a clinical case.