RATIONALE AND OBJECTIVES.Gadolinium III texaphyrin (Gd[III] texaphyrin) complex, a new magnetic resonance imaging contrast (MRI) agent, was evaluated. METHODS.In vitro relaxivity (1.5 T) and stability studies (5% dextrose) were conducted. Subchronic toxicity (8 males, 8 females; 2-20 (µmol Gd(III) texaphyrin complex/kg body weight; 3 times per week for 3 weeks). Biodistribution and excretion studies were conducted in Sprague-Dawley rats; MRI studies were conducted in normal and tumor-bearing rats and rabbits. RESULTS.Relaxivity values were as follows:1=19 (µmol/ L · sec)-1and r2=22 (µmol/L · sec)-1The 21-day subchronic toxicity study revealed no abnormalities. The compound is stable. Biodistribution demonstrated liver uptake. Magnetic resonance imaging in normal (n=34) and tumorbearing (n=4) rats and normal (n=8) and tumor-bearing (n=19) rabbits revealed: significant (P<.05) contrast enhancement of liver and kidney after 1-17 µmol/kg of Gd(III) texaphyrin complex. Gadolinium (III) texaphyrin complex (2.5 (µmol/kg) produced significant contrast enhancement of liver carcinomas in rabbits (n=8). Thigh V2 carcinomas (n=22) had selective (P<.05) enhancement, 5 µmol/kg. In rat fibro-sarcomas (n=4), 17 (µmol Gd(III) texaphyrin complex produced significant enhancement up to 24 hours. CONCLUSIONS.Gadolinium (III) texaphyrin complex appears to be an effective and safe MRI contrast agent.