Acipimox (5-methylpyrazinecarboxylic acid 4-oxide) is a new lipolysis inhibitor that has a distant chemical relationship with nicotinic acid (NA). The tritiated compound (100 mg) is rapidly absorbed, peak plasma radioactivity being reached after 2 hr, with an almost total elimination unchanged in urine. A comparison of th antilipolytic activity of three doses of acipimox and three doses of NA showed acipimox to be 20 times as potent as NA. There was a correlation between intensity and duration of effect for acipimox, but not for NA. Plasma acipimox levels correlated with inhibition of lipolysis. In consideration of the very good subjective tolerability of acipimox at all doses tested, this drug may be suitable for control of lipolysis in hyperlipidemias.