The activity of a renin-like enzyme (RLE) previously found in rat copora lutea was studied during lactation. Luteal RLE concentration significantly increased after delivery and reached a maximum on day 5 of lactation. Plasmatic levels of PRL and progesterone also increased through lactation. Treatment with 2 bromo-alpha-ergocryptine, which diminished plasma PRL and progesterone levels, enhanced luteal RLE activity. Therefore, the increase in luteal RLE during lactation seems to be independent of PRL and progesterone levels, but dopamine could be involved in its regulation. The increase in luteal RLE is not related to the intensity of the suckling stimulus, since RLE values were not modified in mothers suckling 2 to 10 pups. In conclusion, RLE activity in rat corpora lutea changes during lactation with a pattern similar to that of plasmatic PRL and progesterone, but seems not to be regulated by these hormones, nor by the intensity of suckling. On the contrary, luteal RLE may be regulated by dopamine.