Pt and Pd salts possessed either histamine releasing or histamine binding properties in rats. Both these effects were dependent on chemical structure of the tested compounds and might be drawn up in the following sequences: Pd > Pt, PtIV > PtII, NO2 > F > Cl > SCN > J, trans > cis. One of the possible mechanisms of anti-tumoral effect of Pt and Pd salts might be their marked histamine binding activity. [The following compounds were used: K2PtF6, K2PtCl6, K2PtBr6, K2PtI6, K2Pt(NO2)4, K2PtCl4, K2PtBr4, K2PdCl4, K2PdBr4, cis-Pt(NH3)2Cl2, trans-Pt(NH3)2Cl2, cis-Pd(NH3)2Cl2, trans-Pd(NH3)2Cl2, Pt(NH3)4Cl2, Pd(NH3)4Cl2 and K2Pd(NO2)4.cntdot.].