Intradermal injection (1 X 10(6) cells) of rat mammary adenocarcinoma (13762A) was made in the back skin in 12 rats. Tumor growth and characterization was followed with nuclear magnetic resonance imaging (NMRI) in 9 rats (3 rats died before completion of the study) at 3, 4, and 5 weeks after injection, using spin echo, inversion-recovery, and calculated T1 techniques. Three rats were sacrificed after each of the three imaging periods for histological studies designed to distinguish solid tumor mass from necrosis. Qualitative NMR imaging T1 values increased as the tumors increased in size as evidenced by a progressive decrease in image intensity compared to the surrounding tissues on the T1 weighted images. Calculated T1 values also increased as the tumors aged (Week 3 = 0.3 +/- 0.11; Week 4 = 0.45 +/- 0.07; Week 5 = 0.42 +/- 0.03). Planimetry of tumor areas on histological sections showed that as tumors increased in size, the ratio of necrotic area to solid tumor area increased (Week 3 = 0.3 +/- 0.11; Week 4 = 0.45 +/- 0.07; Week 5 = 0.51 +/- 0.05). These findings indicate that the progressive increase in T1 observed on NMR images may be secondary to the increasing degree of necrosis, with a resultant change in water content and state. Thus, the range of T1 values observed in tumors of similar type may be due to change in tumor physiology and anatomy as tumor growth progresses. In conclusion, careful correlation of histological data with NMR image data is necessary before NMR imaging can be used to provide reliable noninvasive histological information concerning tumor pathology.