Fig. 3. In vivo effect of estradiol (E₂) and E₂ administered in combination with melatonin (mel) on mast cell number (MCN) in the frog Rana esculenta. All the animals were injected with E₂ 24 h before being sacrificed and were killed at 48 h, 72 h and on day 8 after the injection of melatonin or Krebs Ringer bicarbonate (KRB) buffer (control). Values are expressed as means ± S.D. Different letters indicate significance (a versus b, c, d, P<0.01; b versus d, e, P<0.01; b versus c, P<0.05; c versus d, e, P<0.01).