A somatic cell mutant (Mev-1) auxotrophic for mevalonate by virtue of a complete lack of detectable 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthase activity has been shown to demonstrate a requirement for a non-sterol mevalonate-derived product for regulation of synthesis of HMG-CoA reductase. A comparison of the effects of 25-hydroxycholesterol and the combination of 25-hydroxycholesterol and mevalonate on HMG-CoA reductase activity, synthesis, and mRNA levels in Mev-1 is presented in this report. The results show a close correlation between activity, rate of synthesis, and mRNA levels for Mev-1 cells treated with 25-hydroxycholesterol alone. Under the conditions of these experiments these effects are relatively small (approximately a 4-fold decrease). A much larger inhibition of HMG-CoA reductase activity and rate of synthesis (approximately 50-fold) is observed upon treatment of Mev-1 cells with a combination of 25-hydroxycholesterol and mevalonate. Yet, under these conditions mRNA levels are still reduced by only a factor of 4. These results are interpreted to suggest that the non-sterol mevalonate-derived regulatory product of HMG-CoA reductase acts by a translational control mechanism.