Pregnant Squalus acanthias were injected intravenously with aqueous extract equivalent to three pituitary ventral lobes (VL). Blood samples were withdrawn by caudal sinus puncture at intervals over a period of 72 hr and plasma progesterone, testosterone, and estradiol-17 beta measured by radioimmunoassay. Early pregnancy animals (Stage A) failed to respond, but mid-pregnancy animals (Stage C) responded to VL with significant increases in plasma steroids. Plasma progesterone and testosterone were increased sevenfold and plasma estradiol-17 beta fourfold by 10-24 hr postinjection. These studies demonstrate pregnancy stage-dependent differences in gonadal sensitivity to a pituitary ventral lobe gonadotropic moiety.