A single 300-mg i.p. injection of poloxamer 407 (P-407, also called Pluronic F-127) in rats produces a marked hypercholesterolemia for a minimum of 96 h. The purpose of this investigation was to determine mechanisms by which P-407 causes hypercholesterolemia. The enzyme targeted for investigation is the rate-limiting enzyme in cholesterolgenesis, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase. Injection of P-407 in fasted rats resulted in a significant (p < 0.05) elevation in plasma cholesterol (61.2 +/- 4.2 mg/dl) as soon as 1 h after injection compared with sham-injected controls (50.1 +/- 3.7 mg/dl). Plasma cholesterol (CHO) 24 h after injection of P-407 was 449 +/- 57 mg/dl, with the fastest rate of accumulation occurring from 1 to 12 h (approximately 16.6 mg/dl/h). Over the concentration range of 0-5 mM, P-407 did not appear significantly to affect the activity of HMG-CoA reductase in vitro. However, the enzymatic activity assayed in microsomal fractions isolated from the livers of P-407-injected rats reached a maximum of 262 +/- 42.6 pmol mevalonate/min/mg approximately 15 h after injection, with a subsequent decline to control activity (94.1 +/- 8.7 pmol/min/mg) at approximately 40 h after injection. At 48 h after injection of P-407, the activity of HMG-CoA reductase significantly (p < 0.05) decreased below control values with a mean activity of 9.4 +/- 1.2 pmol/min/mg. The CHO concentrations in hepatic tissue were significantly (p < 0.01) increased at 2 h (3.26 +/- 0.19 mg/g) and 4 h (3.75 +/- 0.38 mg/g) and significantly (p < .01) reduced at 15 h (1.56 +/- 0.19 mg/g) after injection of P-407 compared with tissue CHO concentrations determined in control animals (2.65 +/- 0.18 mg/g). However, the hepatic CHO content appeared to return to control values by 24 h (mean +/- SEM, 2.61 +/- 0.08 mg/g) after injection. These data suggest that the activity of HMG-CoA reductase is regulated by some indirect mechanism(s) after injection of P-407 in rats.