Chronic treatment of rats with haloperidol decanoate (30 mg/kg and 100 mg/kg IM every 4 weeks for 52 weeks) increased [3H] SCH 23390 binding in striatal membranes by 25% and 50% and in frontal cortical membranes by 56% and 125% in 30 and 100 mg/kg haloperidol treatment groups, respectively. These increases in [3H] SCH 23390 binding to the membranes were restored to control levels after ceruletide treatment (100 micrograms/kg IP twice a day for 5 days). [3H] Spiperone binding to the rat striatal and cortical membranes also increased after chronic haloperidol treatment (by 66% and 99% in striatal membranes and by 27% and 62% in cortical membranes in the 30 and 100 mg/kg haloperidol treatment groups, respectively). Administration of ceruletide to haloperidol-treated rats reduced the increased [3H] spiperone binding to the cortical membranes toward the control level, but ceruletide was not effective in reducing the haloperidol-induced increase of [3H] spiperone binding to the striatal membranes. Activation of adenylate cyclase by dopamine (1 microM or 100 microM) or Gpp(NH)p (1 microM) was reduced in striatal and cortical membranes from haloperidol-treated rats. Ceruletide restored the lowered level of dopamine-stimulated or Gpp(NH)p-stimulated adenylate cyclase activity in the membranes from haloperidol-treated rats to control levels.(ABSTRACT TRUNCATED AT 250 WORDS)