Analgesic effect induced by chronic intravenous injection of AN81 and BVD03 opioid peptides at a dose of 4 mg/kg. The examined drugs were administered daily for five days (between 10 and 12 am). The determination of nociceptive responses was done on the first (Figure 3A) and fifth day (Figure 3B) of the experiment. %MPE ± SEM of 7-10 animals per group. On the first day of measurements BVD03, but not AN81, show significant differences (**p < 0.01) when compared to morphine. Additionally, # represents differences which occur between examined compounds compared to each other. In contrast, on the fifth day statistics do not show any significant differences (*p < 0.05; **p < 0.01;***p < 0.001).