Magnetic resonance imaging (MRI) has been used for the determination in vivo of rat pituitary size. In midsagittal T2-weighted sections the pituitary, having a lower T2 value than the surrounding tissue, was visible with pronounced contrast. The size has been estimated by pixel counting. A close correlation (r = 0.96) with the pituitary weights determined postmortem has been obtained, demonstrating the reliability of the in vivo method. Using MRI the effects of Sandostatin, a somatostatin analog, on the pituitary size have been monitored in a rat model of prolactinoma (estradiol-induced hyperplasia of the pituitary). Treatment with Sandostatin over 4 weeks resulted in a 40% reduction of the hyperplastic pituitaries. These results have been confirmed by determination of pituitary weights postmortem. However, due to a large interindividual variation in size of hyperplastic pituitaries, more animals are required to reach statistical significance when only endpoints of treatment can be measured. In contrast, MRI allows one to monitor individually the drug effects over a long period of time, eliminating interindividual variations.