The effects of separation and suckling on serum growth hormone (GH) levels of lactating rats and their pups were studied on days 1-3 of lactation. The litter size was adjusted to 8 pups/dam without respect to sex. The separation of pups from their mother for 5 h resulted in significant decrease in GH level in the pups. After 30 or 60 min of suckling the level of GH in serum of the pups reached the values of the control group, i.e. non-separated pups. If the pups were attached to pre-suckled mothers, thus consuming less milk, their serum GH levels increased in a significantly lesser extent. It is concluded that some factor(s) are present in milk which stimulate(s) GH secretion of the pups. Due to episodic secretion, high standard error occurred in every group of mothers studied. Mean GH level of freely behaving lactating rats were rather constant. On the other hand, the 5 h separation period and 30 min of suckling seemed to result in decrease and increase in serum GH values, respectively. By the end of a 60 min suckling period, GH declined to low levels again, hence it is likely that the separation and suckling stimuli synchronized GH-episodes. When the litter was replaced with hungry foster pups at 30 min of suckling, GH-decay by 60 min appeared to be less consequent.