Procaine is a local anesthetic, also used in experimental gerontology and has been tested in cultivated human WI-38 fibroblasts. This molecule was found to enhance growth rate and cell densities in actively dividing cultures. As the cells aged, however, this stimulatory effect diminished and finally vanished. In a long term experiment the enhancement of growth of procaine treated cultures was finally replaced by a toxic effect even at low concentration. The amount of the thermolabile enzyme found in phase III cells did not change when procaine was added to the culture medium. In this cellular aging model, procaine behaved like a metabolic stimulator of actively dividing cells but not as an "antiaging" molecule as it is sometimes assumed.