Abstract— We have in the present study investigated the properties of mevalonate kinase, phosphomevalonate kinase and pyrophosphomevalonate decarboxylase in the 105,000 g supernatant fractions from rat brain, and determined whether the activities of these enzymes change during brain development. All three enzymes in brain showed a specific requirement for ATP for optimal activity. The presence of Mg2+ as divalent cation was also required for optimal activity of mevalonate kinase and phosphomevalonate kinase. Both Mg2+ and Mn2+ were equally effective divalent metal ions for pyrophosphomevalonate decarboxylase in brain. Mevalonate kinase as well as phosphomevalonate kinase were active in a broad pH range of 6.5–8 while the pH curve for pyrophosphomevalonate decarboxylase showed a peak activity at approx 6. No age-dependent change occurred in the activities of mevalonate kinase and phosphomevalonate kinase in developing brain, whereas pyrophosphomevalonate decarboxylase activity in brain increased during the 1st week after birth, reached a peak value at about the 8th day of age and declined slowly thereafter. The Km for brain mevalonate kinase in 2, 13 and 52 day old rats were 312, 400 and 434 μM, respectively. The Vmax for the kinase in 2, 13 and 52 day old rats were in the range of 45–52 nmol/h/mg protein, respectively. This suggests that, like in liver (Ramachandran & Shah, 1976), pyrophosphomevalonate decarboxylase in brain may also be one regulatory step for cholesterol synthesis.