Abstract 1. 1. Treatment of Tetrahymena pyriformis cells with diiodotyrosine (T 2) gave rise to a considerable, concentration-dependent increase of the growth rate within the range of 10 −15 and 10 −9 M, but did not influence it at the level of 10 −18 M. 2. 2. Re-exposure of the cells 1, 2 and 4 weeks later to the hormone concentrations originally used accounted for a marked increase of growth rate at all hormone levels tested, indicating that the extremely low concentration of 10 −18 M, which failed to stimulate growth on first exposure, did nevertheless give rise to hormonal imprinting, which caused the cells to “remember” the hormone, as judged from their increased responsiveness to it on re-exposure. 3. 3. The degree of growth response was concentration-dependent on both first and second exposure: higher levels of treatment gave rise to firmer imprinting, and to greater response on re-exposure. 4. 4. The length of exposure time proved to be more decisive than the level of treatment in respect of the development of hormonal imprinting. 5. 5. Short-term exposures up to 60 min, although they stimulated cell growth by direct effect, gave rise to lasting inhibition of cellular response to re-exposure(s) rather than to hormonal imprinting.