There has been increasing interest in studying the various effects of organophosphate insecticides in humans and experimental animals. Only a few data are available on the effect of the organophosphate insecticide, diazinon, on lipid metabolism. The aim of this study was to evaluate the effect of diazinon on plasma lipid constituents in mammalian animals. The plasma levels of total cholesterol (TC), high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), triglycerides (TG), and phospholipids (PL) were measured in albino rats that were orally treated with a single dose of diazinon at a level of $LD_{50}$ or with repeated daily doses at the levels of $frac{1}{2}$, $frac{1}{8}$, and $frac{1}{32}$ $LD_{50}$ for 2, 8, and 32 days, respectively. After a 24 h post-treatment with a single $LD_{50}$ dose of diazinon, TC was not significantly changed, the HDL-C and PL levels were significantly decreased, but the LDL-C and TG levels were significantly increased. Separate daily oral administrations of diazinon at $frac{1}{2}$ $LD_{50}$, $frac{1}{8}$ $LD_{50}$, and $frac{1}{32}$ $LD_{50}$ doses resulted in a significant decrease in HDL-C and PL, with no significant change in TG. The LDL-C levels were significantly increased and TC showed no significant change with $frac{1}{2}$ $LD_{50}$ and $frac{1}{32}$$LD_{50}$ doses of diazinon, whereas a significant decrease in the levels of TC, HDL-C, as well as LDL-C, was observed with the $frac{1}{8}$ $LD_{50}$ dose. These data suggest that diazinon may interfere with lipid metabolism in mammals.