The microstructure of polyisoprenes prepared in hydrocarbon solvents with n-butyllithium is very sensitive to pressure, whereas in oxygenated solvents the structure remains unchanged even in the highest pressure range. Thus in n-heptane the content of 1,4-cis units decreases with increasing pressure, leading to polymers with heterogeneous structure. It is assumed that electrostatic as well as steric effects must be taken into account together: pressure acts like a polar solvent; however, in the transition state, steric effects affect the 1,4-cis addition.