Pregnant female spiny dogfish were captured off the coasts of Maine and Massachusetts and morphometric and gravimetric analyses of the liver, gonad, and reproductive tract correlated with the reproductive cycle. Plasma samples were taken and circulating steroids (progesterone, testosterone, and estradiol) were measured by radioimmunoassay. Clear-cut temporal variations in plasma steroid levels were observed, which correlated with the ovarian cycle. Testosterone and estradiol-17 beta levels were low (100-200 pg/ml plasma) in early pregnancy when ovarian follicles were small but increased to about 10 ng/ml as follicles grew and gestation progressed. By contrast, plasma progesterone was high (2-6 ng/ml) in early to mid-pregnancy when corpora lutea were active and declined toward term (less than 1.0 ng/ml).