The regulation of mevalonic acid synthesis requires both nonsterol isopentenoid and sterol regulatory signal molecules. A primary target of this multivalent control process is the enzyme which catalyzes mevalonate synthesis: 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase (EC 1.1.1.34). In this report Staphylococcus aureus alpha-toxin perforated Chinese hamster ovary cells were used to facilitate the identification of isopentenoidogenic reactions and metabolites required for mevalonate-mediated loss of HMG-CoA reductase activity. alpha-Toxin-perforated cells retained the capacity to decrease, upon demand, HMG-CoA reductase activity and protein in response to mevalonate or isopentenoid pyrophosphate esters. Also, it was deduced with highly specific metabolic inhibitors, that conversion of farnesyl 1-diphosphate to squalene was required for mevalonate-mediated suppression of reductase activity. Since squalene (2 microM) did not downregulate reductase activity, pre-squalene pyrophosphate or a derivative, or polyprenyl-1-pyrophosphate-generated inorganic pyrophosphate, or a combination of these metabolites are proposed as candidate regulatory nonsterol isopentenoid signal molecules.