Severed aphid stylets were used to follow the kinetics of sieve tube turgor and osmotic pressure (pi) responses following step changes in water potential applied to the cambial surface of willow (Salix exigua Nutt.) bark strips. The kinetics of the turgor response were monitored with a pressure transducer. In separate experiments, the kinetics of the pi response were followed by freezing point determinations on stylet exudate. The sieve tube volumetric elastic modulus in the bark strips was about 21 bars, but may be higher in intact stems. The membrane hydraulic conductivity was about 5 x 10(-3) centimeters per second per bar; several factors make it difficult to estimate its value accurately. Differences in the turgor pressure (P) and pi responses, as well as the relatively more rapid initial turgor response to a water potential (psi) change, suggested a time-dependent component in sieve tube wall elasticity.Our observations were generally not supportive of the idea that sieve tubes might osmoregulate. However, the bark strip system may not be suitable for addressing that question.Separate measurements of psi, P, and pi demonstrate that the relationship predicted by the fundamental cell water potential equation, psi = P - pi, is applicable within experimental error (+/- 0.4 bar) to sieve tube water relations.