No correlations between the three hormones measured were found in any of the reproductive states examined. In 2 out of 8 dogs a negative correlation between serum progesterone and prolactin levels was found at Day 11 and Day 15, respectively (Day 0 = first day of copulation). In all of the 8 animals in which the progesterone and prolactin concentrations were measured in the same samples in late pregnancy, a negatively correlated overall pattern was obtained between both hormonal profiles. No correlation between serum oestrogen and prolactin levels was found. It is concluded that the proliferative mammary gland changes which are induced in female dogs during long-term toxicity testing with progestagens are unlikely to be related to prolactin or oestrogen synthesis and/or secretion.