In vitro nickel (II) complex presents antimitotic effects. In this work, we have studied the in vivo seasonal effects of nickel (II), ligand and the complex [NiCl2(M5FTSC)2] in male Swiss mice. During spring, an intra peritoneal (i.p.) injection of NiCl2 in aqueous NaCl up to 1.0.10-2 mmol.kg-1 body weight (b.w.) killed 10% of the rodents after 6 days. Lethal dose 100% (LD100) was up 1.91.10-2 mmol.kg-1 b.w.; ligand was less toxic than Ni (II), while the complex was 25% lethal at 1.37.10-2 mmol.kg-1 b.w. In autumn, mice were less sensitive to NiCl2. The ligand and the complex, on the contrary, were more toxic. This leads us to the conclusion that, in vivo, chronotolerance of nickel (II), ligand and complex in aqueous solution, are quite different in spring and autumn seasons.