While studying the environmental fate of potent endocrine-active steroid hormones, we observed the formation of an intramolecular [2+2] photocycloaddition product (2) with a novel hexacyclic ring system following photolysis of altrenogest (1). The structure and absolute configuration were established by X-ray diffraction analysis. Theoretical computations identified a barrierless two-step cyclization mechanism for the formation of 2 upon photoexcitation. 2 exhibited progesterone, estrogen, androgen, and pregnane X receptor activity, albeit generally with reduced potency relative to 1.