Studies of the effect of anesthetic molecules, halothane and isoflurane, on the aggregation behavior of POE-POP-POE triblock copolymers, P84, F88, P104, in aqueous solution,have been carried out using fluorescence spectroscopy and light-scattering techniques. The hydrodynamic radius of block copolymer aggregates and the I/III vibronic intensity ratio of pyrene in 5 wt % aqueous solutions of F88, P84, and P104 were obtained over a wide temperature range, 10-60 degrees C. Large-sized clusters form at low temperatures for P104, but they do not have a well-defined hydrophobic core. Low concentrations of the anesthetics are found to induce the aggregation of F88 and P84 at 25 degrees C, and this effect is similar to that of increasing the temperature. The effect of halothane and isoflurane concentration on the change of hydrodynamic radius of copolymer aggregates and the I/III vibronic ratio of pyrene in F88, P84, and P104 were investigated at 25 degrees C. The addition of 1 mM of anesthetic is equivalent to ca. a 1-2 degrees C increase in temperature. It is deduced that the anesthetic molecules are distributed in both the hydrophobic core and hydrophilic corona of the polymer micelles. However, whereas an increase in temperature removes water of hydration from around the POP and POE segments, the anesthetic molecules tend to replace water of hydration around these segments. It is inferred that anesthetic molecules could disturb the water of hydration around biological macromolecules, and it is proposed that the dehydration effect of inhalation anesthetics may play an important role in the process of anesthesia.