PURPOSE: To determine the efficacy of Gd-ethoxybenzyl-DTPA(Gd-EOB-DTPA) in MR imaging of primary hepatictumors in rats. MATERIALS AND METHODS: Primary hepatic tumors had been induced on 12 male Sprague-Dawley rats byoral administration of an experimental diet containing 3'-methyl-4'-dimethylaminobenzene for 12 weeks. T1-weighted spin echo images were obtained with 1.5 T system before and after injection of 25 micro mol/kg ofGd-EOB-DTPA. The detection rate of tumors in each image was compared to that in resected specimens. Signal intensity of the liver and the largest tumor were measured in each rat on pre- and postcontrast images. Signal tonoise ratio(SNR) of the liver and liver-to-tumor contrast to noise ratio(CNR) were calculated. RESULTS: One hundred and twenty-five liver tumors were identified on gross examination. Fifty one lesions(41%) on precontrast images, and 80(64%) on postcontrast images were demonstrated (p<0.01). Of the 33 tumors larger than 5mm, 28lesions(85%) were demonstrated on precontrast image, and 31(94%) on postcontrast image (p>0.05). Of the 92 lesions smaller than 5mm in diameter, 23(25%) were seen on precontrast image and 49 the (53%) on postcontrast image(p<0.01). After contrast enhancement, the signal intensity of both liver and tumor increased(p<0.05), though background noise also increased (p>0.05). Consequently, SNR increased significantly in the liver from 10.89 ±1.96(mean standard error) to 15.59 ± 2.34(p<0.05), but not in tumors(from 8.21 ± 3.71 to 8.85 ± 5.42). Liver-to-tumor CNR increased from 5.55 ± 0.94 to 13.25 ± 2.50 (p<0.01). CONCLUSION: The use of Gd-EOB-DTPA canincrease the detection rate of primary hepatic tumors in rats, especially of small lesions.