The effects of acute infusion of lithium chloride (LiCl) were studied on mean arterial pressure (MAP) and magnocellular activity as shown by the concentrations of vasopressin-associated neurophysin ([VP-RNP]) and concentrations of oxytocin-associated neurophysin ([OT-RNP]) in plasma in conscious Long-Evans rats. Chronically-cannulated rats were infused intravenously at 10 microliter/100 g body wt/min with 13% LiCl for 20 min (total dose = 6.16 mequiv./kg body wt) or 0.65% LiCl for 60 min (total dose = 0.92 mequiv./kg body wt). Effects of 13% LiCl on mean arterial pressure were also examined in vasopressin-deficient homozygous Brattleboro rats. For Long-Evans rats, infusion of 13% LiCl produced rapid and significant (P less than 0.001) increases in mean arterial pressure, the concentration of lithium in plasma ([Li+]), plasma osmolality (Posmol), [VP-RNP], [OT-RNP] and significant decreases in heart rate and sodium concentration in plasma ([Na+]). For similar changes in plasma osmolality, lithium had a greater effect than sodium on mean arterial pressure, [VP-RNP], [OT-RNP]. For the 20 min infusion of 13% LiCl, there was a significant relationship (P less than 0.033) between delta MAP and log delta[VP-RNP] with a slope of 11.9 mmHg fmol-1 ml-1 (r = 0.5678). Unlike that of Long-Evans rats, infusion of 13% LiCl only did not produce significant changes of mean arterial pressure in Brattleboro rats. For Long-Evans rats, infusion of 0.65% LiCl resulted in more gradual and smaller elevations of blood pressure, [Li+] and smaller decreases in heart rate with no significant changes in plasma osmolality, [Na+], [VP-RNP] and [OT-RNP].(ABSTRACT TRUNCATED AT 250 WORDS)