Extracts from brains of mouse, rat, rabbit, pig, chicken, and duck were assayed for peptidylarginine deiminase, which catalyzes the deimination of arginyl residues. There was peptidylarginine deiminase in the extracts from all the brains that we tested, suggesting the widespread occurrence of this enzyme in vertebrate brains. The levels of the activity in the brains of 8 different inbred strains of mice could be divided into two groups. There is a difference of over 300% between the high and low groups. Furthermore peptidylarginine deiminase purified from the brains of mouse, pig, and chicken showed that there is no great difference in their MW and the optimal conditions for the activity. However, the substrate specificity of the enzyme from chicken brain was somewhat different from those of the enzymes from mammalian brains.