The effects of ovine prolactin (PRL) (2 x 5 IU a day) and an ectopic pituitary isograft on the responsiveness were examined using locomotor and exploratory activities as measures in PRL-growth-hormone-thyrotropin-deficient Snell dwarf mice (dw/dw). After 5 weeks of treatment, both PRL and the graft restored the two behavioural measures to normal levels. Results clearly demonstrate the involvement of PRL in global behavioural responsiveness and suggest a possible role for PRL in the changes induced by the graft.