Summary Diseases associated with acidotic blood-pH, such as chronic renal disease, diabetes mellitus or chronic alcoholism, show a marked impairment of lipoprotein lipase. Therefore we influenced blood-pH in 3 healthy subjects by infusions to get alkalotic, neutral and acidotic blood-pH on three days in series. On each day blood-pH from capillary blood and post-heparin lipoprotein lipase from fasting plasma was determined. In comparison to neutral blood-pH in vivo, alkalosis did not influence lipoprotein lipase. In contrast, during artificial acidosis, lipoprotein lipase was impaired significantly (p<0.01). Therefore, it seems, that acidosis inhibits lipoprotein lipase in vivo.