The leakage of lactose from the mammary gland into plasma and the increase in the suckling-induced concentration of prolactin in the plasma of oxytocin-injected lactating rats suckling different numbers of pups were investigated. Rats nursing eight pups were isolated from their litter for 6 h and injected i.v. with either 1 iu oxytocin or saline and allowed to resuckle either two, eight or 12 pups for 1 h. The concentration of lactose in plasma increased significantly in rats injected with oxytocin and allowed to resuckle two or eight pups; however, there was no difference in rats allowed to resuckle 12 pups. When compared within oxytocin-injected groups, the concentration of lactose was significantly lower in rats allowed to resuckle eight or 12 pups than in rats allowed to resuckle only two pups. Plasma prolactin concentrations increased during the suckling period in mothers suckling either eight or 12 pups, and oxytocin diminished the increase in prolactin concentration in both groups. In rats suckling two pups, prolactin release was not observed. The second objective of this study was to determine the relationship between the leakage of lactose from the mammary gland by oxytocin injection and suckling. Mothers that had the three left nipples of their mammary glands occluded were isolated from their litter of eight pups for 6 h and were allowed to resuckle with eight pups for 1 h. At the start of resuckling, 0.01, 0.1 or 1 iu oxytocin was injected i.v.(ABSTRACT TRUNCATED AT 250 WORDS)