Developmental exposure to the organophosphorus pesticides chlorpyrifos and diazinon (DZN) alters serotonergic synaptic function at doses below the threshold for cholinesterase inhibition, however there are some indications that the two agents may differ in several important attributes. Previously, we found that low-dose chlorpyrifos exposure in neonatal rats causes lasting changes in emotional response and in the current study we did a comparable evaluation for DZN. Male and female Sprague–Dawley rat pups (N =10–12 of each sex per treatment group) were given 0, 0.5 or 2mg/(kgday) of DZN s.c. daily on postnatal days (PND) 1–4. These doses bracket the threshold for barely-detectable cholinesterase inhibition. Starting on PND 52, these rats began a battery of tests to assess emotional reactivity. In the elevated plus maze, there was a slight decrease in the time spent in the open arms for DZN-exposed males, while DZN-exposed females were not different from control females. In the novelty-suppressed feeding test, DZN-exposed males had significantly shorter latencies to begin eating than did control males, reducing the values to those normally seen in females. DZN-exposed rats of either sex showed reduced preference for chocolate milk in the anhedonia test that compared the consumption of chocolate milk to water. These findings show that neonatal exposures to DZN at a dose range below the threshold for cholinesterase inhibition nevertheless evokes specific, later alterations in emotional behaviors, particularly in males. The effects show not only some similarities to those of chlorpyrifos but also some differences, in keeping with neurochemical findings comparing the two agents.