The effect of 10 low molecular mass alkanols on the activity of Ca-ATPase (EC 3.6.1.38), Ca uptake and Ca efflux as well as the functional efficiency of the Ca pump in the fragmented sarcoplasmic reticulum of rabbit skeletal muscles has been studied. Some alkanols, especially when taken at low concentration, have been found to stimulate the activity of the Ca pump and Ca-ATPase, namely tert-butanol, isopropanol and ethanol (from the group of hydrophilic alkanols), and pentanol, isopentanol and hexanol (from the more hydrophobic alkanols). Methanol (from the first group) and isobutanol, butanol and propanol (from the second) do not stimulate the Ca pump compared with the control. The specific effect of different alkanols cannot be explained in terms of a unitary mechanism based on 'fluidity' changes of the membrane. It is assumed that, at low concentrations, certain alkanols (or groups of related alkanols) are able to promote the specific transition of membrane proteins into the active state, whereas at higher concentrations all alkanols provide for the non-functional state of the proteins.