THE EXCLUSIONARY RULE, WHICH WAS IMPOSED ON THE STATES BY THE SUPREME COURT IN MAPP V. OHIO IN 1961, PROHIBITS THE ADMISSION IN CRIMINAL TRIALS OF EVIDENCE SEIZED IN VIOLATION OF THE FOURTH AMENDMENT. TWENTY-TWO STATES HAD ADOPTED THEIR OWN EXCLUSIONARY RULES PRIOR TO THE SUPREME COURT MANDATE. BECAUSE DATA ON ILLEGAL SEARCHES ARE UNAVAILABLE, ARREST RATES ARE USED AS AN INDIRECT MEASURE OF THE IMPACT OF THE MAPP DECISION, THE ASSUMPTION BEING THAT REDUCTIONS IN ILLEGAL SEARCHES WOULD BE REFLECTED IN REDUCED ARREST RATES. ARREST DATA ARE ANALYZED FOR OFFENSES THAT OFTEN GIVE RISE TO SEARCH-AND-SEIZURE ISSUES. THE UNIT OF ANALYSIS IS THE CITY-OFFENSE COMBINATION; THE TIME FRAME IS 4 YEARS BEFORE AND AFTER THE MAPP RULING. ALTHOUGH DATA LIMITATIONS PRECLUDE A PRECISE DETERMINATION OF LAW ENFORCEMENT RESPONSE, THE FINDINGS INDICATE THAT THE MAPP RULING HAS INFLUENCED THE ENTIRE SEARCH-AND-SEIZURE BEHAVIOR OF SOME DEPARTMENTS WHILE NOT AT ALL AFFECTING THE BEHAVIOR OF OTHERS. THE DECISION APPEARS TO HAVE HAD AN IMPACT IN AN APPRECIABLE MINORITY OF CITIES AND CITY-OFFENSE COMBINATIONS, BUT NOT IN THE MAJORITY. THERE IS NO DRAMATIC DIFFERENCE IN POLICE RESPONSE BETWEEN STATES THAT ALREADY HAD AN EXCLUSIONARY RULE AND THOSE THAT DID NOT. THERE IS REASON TO INFER THAT THE FEDERAL RULING HAS HAD A GREATER IMPACT ON ILLEGAL POLICE SEARCHES THAN DID STATE RULINGS. POSSIBLE EXPLANATIONS OF THE OBSERVED VARIATIONS IN POLICE RESPONSE TO THE MAPP RULING ARE CONSIDERED, TOGETHER WITH WAYS OF ENHANCING THE STATES' IMAGE AS A PROPER SOURCE OF CIVIL LIBERTIES POLICY. SUPPORTING DATA ARE INCLUDED. (LKM)