Time course of the analgesic effects of morphine, chlorpromazine, chlordiazepoxide, and buprenorphine after systemic administration in amphibians (n = 4 to 6 per group).○, morphine (300 nmol/g); ▽, chlorpromazine (100 nmol/g);■, chlordiazepoxide (300 nmol/g); ⋇, buprenorphine (30 nmol/g). [“Key” deleted from illustration] Saline-injected control group did not give a significant effect and is omitted for clarity.