Previous studies have shown that the ratio of lipoprotein lipase (LPL) catalytic activity to LPL mass in tissues differs in different conditions, but it is not clear whether this occurs by a change in the catalytic efficiency of the LPL molecules, or because of a shift in the relation between active and inactive forms of the enzyme. To explore this, we have measured LPL activity and mass in detergent extracts of rat tissues. LPL specific activity was high and similar in heart, skeletal muscle, lung and brain. The liver had significantly lower specific activity, which is in accord with previous findings that the liver takes up and catabolizes LPL. The specific activity was also low in adipose tissue from fasted rats. When tissue extracts were applied to columns of heparin–agarose and eluted by a gradient of NaCl, a peak of active LPL was eluted at 1.0 M NaCl, but there was also a peak of inactive LPL protein, which was eluted at 0.6 M NaCl. In adipose tissue, LPL activity decreased by 70–80% during an overnight fast, whereas LPL mass decreased by only 20–40%. The mass ratio between inactive and active LPL, as separated by heparin–agarose chromatography, increased from 0.5 to over 2 during the fast. In hearts there was no significant difference between fed and fasted rats in total LPL activity, LPL mass or in the distribution between inactive and active forms. The results indicate that the relation between inactive (probably monomeric) and active (dimeric) forms of LPL is a target for post-translational regulation in adipose tissue.