AbstractThe level of calcium‐activated neutral protease (CANP) activity in the brain and nerve cord of the crayfishProcambarus clarkiiwas assayed by measuring the degradation of casein yellow by tissue homogenates. When care was taken to maintain the ionic strength of all incubation media at 0.15Mand to buffer the Ca2+activity of the media with 5mMEGTA, CANP was found to be very sensitive to Ca2+; maximal activity was achieved at 1 × 10−3MCa2+, with 50% of this maximum present at the physiologic intracellular Ca2+activity of 1 × 10−7M. We found that the anticonvulsant agent phenytoin was without effect on CANP activity while pentobarbital and a relatively new anticonvulsant agent, valproic acid (an eight‐carbon branched fatty acid), significantly inhibited CANP activity in a dose‐dependent manner. The inhibitory effect of valproic acid was shared by a straight‐chain eight‐carbon fatty acid, caprylic acid. These findings demonstrate that inhibition of CANP activity is not limited to agents with a specific molecular structure. They also suggest that CANP plays a role in the normal turnover of proteins that control various cel