Plasma immunoreactive insulin levels, determined by radioimmunoassay, show a biphasic pattern of response to avoidance sessions in the monkey. There is an initial drop in plasma immunoreactive insulin levels during the first hours of avoidance and a later prolonged elevation of levels above baseline--persisting often for many days following 72-hr. avoidance sessions. The fact that the plasma insulin changes in avoidance do not appear to be mediated by changes in plasma glucose levels suggests that other regulatory influences on insulin secretion may supersede blood sugar regulation of insulin levels under some conditions. Some possible mediating mechanisms underlying the observed suppression of plasma insulin levels are discussed. Some evidence is presented that emotional reactions to venipuncture may also elicit a decrease in plasma immuno-reactive insulin levels with associated elevations in plasma 17-OHCS and glucose levels.