Experiments were done to study the effects of porcine relaxin on osmotically evoked changes in intramammary pressure and the release of oxytocin and vasopressin in anaesthetized rats. Injections (1 microliter) of hypertonic (0.75 M) NaCl into the left lateral cerebral ventricle were used to induce consistent rises in intramammary pressure and the release of oxytocin and vasopressin. Plasma hormone concentration was determined by radioimmunoassay. Relaxin (5 micrograms i.v.) significantly (P < 0.05) suppressed the intramammary pressure response to osmotic challenge 5 and 10 min after treatment. However, pretreatment with a specific vasopressin V1 receptor antagonist completely negated the effect of relaxin on intramammary pressure. Baseline levels of oxytocin and vasopressin in unstimulated rats were 41 +/- 1.6 and 36 +/- 1.1 pmol/l respectively. Osmotic challenge induced significant (P < 0.05) rises in plasma levels of both hormones (62.8 +/- 1.1 and 67.9 +/- 1.2 pmol/l respectively) which were further augmented by relaxin (81.3 +/- 1.8 and 117.1 +/- 2.4 pmol/l respectively; P < 0.05). The data confirm that central osmotic challenge provokes the release of oxytocin and vasopressin but the effects of oxytocin at the level of the mammary gland may be obscured by the action of vasopressin affecting blood flow to the gland.