We have examined the changes in cellular contractility and adhesive morphology that accompany the reverse transformation of CHO cells caused by dibutyryl cyclic adenosine monophosphate. The ability of these cells to distort elastic silicone rubber substrata was found to increase markedly over the 5 h period following addition of cyclic AMP and testosterone to the medium, and to relax again following hormone removal. Parallel changes were observed in the interference reflexion image of the adhesion of these cells to glass substrata; broad grey-appearing 'close' contacts characteristic of the transformed state were gradually replaced by numerous small black-appearing 'focal contacts'. These changes in adhesive morphology required considerably longer (20-25 h) to be completed or to revert, however.