The effects of insulin and dexamethasone on the secretion of lipoprotein lipase (LPL) by mouse peritoneal macrophages were examined in vitro. Macrophages from either normal or thioglycollate-primed mice continuously secreted LPL into the culture medium. The time courses and the amounts of enzyme secretion and the responses to the hormones were essentially the same in resident or thioglycollate-primed macrophages. Insulin did not enhance the secretion of this enzyme by macrophages, even though a marked effect of this hormone on the enzyme in adipose tissue has been well established. Dexamethasone, which has been reported to stimulate the secretion of LPL in the heart, suppressed the secretion of LPL by macrophages. The present study is the first report to deal with the effect of hormones on the secretion of LPL by macrophages, and clearly demonstrates that insulin does not play an important role in the regulation of LPL activity in macrophages. Dexamethasone also showed a different effect on macrophage LPL compared to that on the enzyme in other tissues. This difference in the regulation of LPL may be relevant to the possibly different role of this enzyme in macrophages as compared to other tissues such as adipose tissue, muscle, or heart.