The proliferation of 3T6 cells was substantially decreased when the monolayer cultures were allowed to reach confluency. This growth inhibition (so-called density-dependent inhibition) was of the same magnitude as that following serum depletion in non-confluent cultures. Each type of growth inhibition was correlated to a depression of the activity of 3-hydroxy-3-methyl-glutaryl coenzyme A (HMG CoA) reductase, an enzyme that regulates the biosynthesis of cholesterol and isoprenoid derivatives (e.g. dolichol) by catalysing the reduction of HMG CoA (which is derived from acetyl-CoA) into mevalonate. However, the depression of enzyme activity was more substantial in cells exposed to cell crowding than that in serum-depleted cells (87 and 48%, respectively). On the other hand, there was a 60-65% inhibition of the incorporation of mevalonate into dolichol due to serum deprivation, while it remained at normal level in confluent cultures, which implies that the inhibitory effects on dolichol synthesis due to these two experimental conditions were approximately equipotent. Addition of epidermal growth factor (EGF) to the cell cultures, whose proliferation was inhibited due to serum depletion, restored DNA synthesis completely, and these effects were related to a normalization of the activity of HMG CoA reductase and of the incorporation of mevalonate into dolichol. In contrast, in confluent cells addition of EGF only caused a slight increase in DNA synthesis and activity of HMG CoA reductase, and there was no significant increase in the incorporation of mevalonate into dolichol either.(ABSTRACT TRUNCATED AT 250 WORDS)