Mono-ADP-ribosylation is a reversible modification of proteins with NAD:arginine ADP-ribosyltransferases and ADP-ribosylarginine hydrolases catalyzing the forward and reverse reactions, respectively. Hydrolase activities were present in a variety of animal species, with the highest specific activities found in rat and mouse brain, spleen, and testis. Rat and mouse hydrolases were dithiothreitol- and Mg(2+)-dependent, whereas the bovine and guinea pig enzymes were dithiothreitol-independent. A rat brain hydrolase was purified approximately 20,000-fold and represented the major approximately 39-kDa protein on denaturing gels. Immunoaffinity-purified rabbit polyclonal antibodies reacted with 39-kDa proteins from turkey erythrocytes and rat, mouse, and calf brains. A rat brain cDNA library was screened using oligonucleotide and polymerase chain reaction-generated cDNA probes. Inserts from two overlapping clones yielded a composite sequence that included a 1086-base pair open reading frame, which contained amino acid sequences found in the purified hydrolase. A hydrolase fusion protein, synthesized in Escherichia coli, reacted with anti-39-kDa polyclonal antibodies and exhibited Mg(2+)- and dithiothreitol-dependent hydrolase activity. A coding region cDNA hybridized readily to a 1.7-kilobase band in rat and mouse poly(A)+ RNA, but poorly to bovine, chicken, rabbit, and human poly(A)+ RNA. The immunological and molecular biological data are consistent with partial conservation of hydrolase structure across animal species.