The transition of insulin between its crystallographically defined states T and R is connected with considerable change even of backbone structure: the N-terminal B chain (residues B1-B8) refolds from extended conformation in T into helical in R, and vice versa. Although hitherto observed only in hexamers the transition of the monomer was adequate for developing and testing the method of 'targeted energy minimization' (TEM), capable of coping with conformational changes of such extent at moderate computational expenditure. The simulation is performed in a predetermined number of steps consisting of two atomic displacements each, one by force in the direction of the target structure, the second by energy minimization releasing the constraint caused in the first. The transition pathway is represented by the string of energy minimized transient structures. Due to the directedness of the algorithm the simulated pathway for R-->T is not the reversal of that for T-->R. It is, therefore, not pretended that the minimum energy pathway was identified. In the T-->R direction the N-terminal B chain first swivels while remaining largely stretched and then winds up extending the pre-existing helix B9-B19. The A chain advances into the space abandoned and withdraws from it in the R-->T simulation. In the latter the extended helix first kinks at B8/B9, and then the B1-B8 segment is unwound and stretched. The helical H-bonds of that segment are formed late in T-->R and are maintained during almost half of R-->T. The AN helix is less stable and more involved in the transitions than helix AC.(ABSTRACT TRUNCATED AT 250 WORDS)