[en] Gadolinium hydrogen α, α', α'', α'''-tetramethyl- 1,4,7,10-tetraazacyclododecane- 1,4,7,10-tetraacetate (abbreviated Gd-DOTMA) was developed as a new contrast agent for magnetic resonance imaging. Our study focused on the evaluation of the pharmaceutical properties as in vivo agent. The new modified process by which Gd-DOTMA was synthesized resulted in high yields of this agent. A high stability constant of 10²⁶ fro Gd-DOTMA was determined at physiological pH. It is more stable than Gd complex with tetraazacyclododecanetetraacetic acid (which is regarded as the most stable Gd complex). The strong T₁ relaxivities of 4.0 and 3.7 (mM · s)^-1 at 0.5 tesla and 1.5 tesla were measured in the aqueous solution. The osmolarity of 0.5 M solution, dissolved with equal amounts of meglumine as a solubilizer is 1020 mOsmol/kg. This contrasting agent was studied in vivo by using rats as the experimental group. The agent showed strong enhancement of transplanted tumors within the rat population studied. This compound is rapidly excreted by the kidneys, and has a half-life of 26 min in blood. The median lethal dose (LD₅₀ value) of the stable Gd-DOTMA has a favorable tolerance of over 12.3 mmol/kg. (author)