The effects of i.c.v. administration of diisopropylfluorophosphate on drinking activity, urine volume and urinary electrolyte excretion were examined in the conscious, unrestrained rat. An i.c.v. dose of 5 micrograms of diisopropylfluorophosphate was found to inhibit acetylcholinesterase activity in five nuclei in the anterior, dorsal portion of the hypothalamus. This dose of diisopropylfluorophosphate also resulted in a dipsogenic episode, a diuresis and a biphasic effect on urinary sodium and potassium excretion. Each response was independent of the other two responses. The responses were mediated by muscarinic cholinergic receptors other than nicotinic receptors as they were blocked by both atropine and pirenzepine but not mecamylamine. The results suggest that the muscarinic receptors responsible are probably of muscarinic receptor M1 class.