Tetrahymena cells treated (imprinted) with insulin on a single occasion bound significantly more insulin than the control cells for as many as 70 days, i.e. over 664 generation changes. Although late reexposure to insulin reduced the binding of labeled hormone for 24 h, the binding value of the imprinted cells still increased significantly over the control. Maintenance under anaerobic conditions for 80 days accounted for a temporary suspension of the effect of imprinting which was, however, recovered within a week of return to aerobic conditions, in an even stronger form than observed in cultures maintained without an anaerobic episode. The experiments demonstrated that the imprinting-induced receptor "memory" lasted long, but was vulnerable to treatment with another polypeptide hormone.