Our aim was to check for calcium channel maturation and regulation on newborn rats during breastfeeding by mothers treated with the L-type calcium channel blocker nifedipine. Contractions by KCl and radioligand binding techniques were used to verify if Ca2+ channels are modified in rat vas deferens of 40-day old litters that were breastfed by mothers injected daily with nifedipine during nursery. Injections were applied in the beginning (1st until 8th day), middle (9th until 16th day), or end (17th until 24th day) of nursery, to verify the period of highest susceptibility of newborn to nifedipine receptor regulation. Contractile responses revealed that only after the middle period of treatment of mothers the maximal effects (Emax) induced in pups by KCl were increased by about 35%, without changes of apparent affinity (pD2). Additionally, binding studies with [3H] Isradipine in cell membrane preparations showed a greater density (Bmax) of Ca2+ channels by about 55%, without changes of affinity (Kd). Changes were not detected after treatment of mothers in the beginning or end of breastfeeding. In addition, in vas deferens of 60-day old litters, the Emax returned to control values, showing that changes were not persistent. Moreover, body and vas deferens weights and blood testosterone of newborn were never changed. The histology of mammary gland was similar for treated and control mothers, suggesting a stable milk production. It is concluded that nifedipine treatment of mothers, if made during the 9th to 16th day of lactation, produced a short lasting reversible up-regulation of L-type Ca2+ channels.