Insulin-like growth factor I (IGF-I) is a mitogen for human breast cancer cells both in vivo and in vitro. We demonstrate here that the antiestrogen ICI 182780 (ICI) at 10(-8) m decreases IGF-I receptor (IGF-IR) mRNA levels by 70% after treatment for 48 h. Measurements of mRNA stability indicate that the half-life of IGF-IR mRNA is approximately 3 h. Estradiol treatment increases the half-life of the IGF-IR mRNA to approximately 6 h and the level of IGF-IR gene transcription by 1.8-fold, whereas ICI treatment not only decreases the IGF-IR transcription rate by 50% but also decreases the IGF-IR mRNA half-life to less than 3 h. Affinity labeling studies with [125I]-IGF-I show 35% increased labeled IGF-I to MCF-7 cell membrane following estradiol treatment and 40% decreased labeling following ICI treatment. We also demonstrate that ICI attenuates IGF-I-stimulated growth. Our data suggest that attenuation of IGF-I responsivity by ICI may be due in part to reducing the IGF-IR expression.