Acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) activities were examined in several tissues of normal and hypophysectomized male and female rats. Significant sex differences in the mean AChE activities of normal rats were observed in the superior cervical ganglion (three times more activity in males) and in serum (50% more activity in females). Sex differences in the BuChE activity of serum and liver were even larger (ten times more activity in females), but the activity of other tissues was similar in both sexes. Hypophysectomy had little effect on the mean activity of AChE but did alter BuChE activity in certain tissues. Most of the effects of hypophysectomy on mean BuChE activity were opposite in direction in the two sexes. For example, in males hypophysectomy caused increases in the BuChE activity of serum (300%) and liver (43%), while in females it caused decreases in both tissues (25 and 30% respectively). In rats of a given group, the AChE activity of each tissue appeared to be regulated independently of the activity in other tissues. By contrast, BuChE activity showed statistically significant correlations in more than half of the tissue-pairs examined in control rats of either sex. These correlations can be considered to reflect a tendency toward body-wide regulation. In female rats, the cross-tissue correlations were largely eliminated by hypophysectomy. This finding indicates that the regulation of BuChE may be strongly affected by hormones under the control of the pituitary gland. However, in male rats, only the correlations involving atria were altered by hypophysectomy. Therefore, the effects of hormones on BuChE are probably both sex and tissue dependent.