The involvement of protein phosphatases and peptidyl-prolyl cis/trans isomerases (PPIases) in memory formation in the chick has previously been investigated using a single-trial learning task. In these studies, inhibitory agents were administered bilaterally directly to a critical area of the chick brain. These studies are now extended to investigate whether similar effects are obtained if the drugs are administered unilaterally. All of the effects reported previously following bilateral administration of okadaic acid (OA), cyclosporin A (CyA), FK506 and [MeVal(4)]CyA can be attributed to their action in just one hemisphere. OA, at a concentration known to selectively inhibit PP2A in vitro (0.5 nM) results in permanent memory loss from 30-40 min post-training when injected in the left hemisphere, but has no effect when injected in the right hemisphere. A higher concentration of OA (100 nM), which inhibits both PP2A and PP1 in vitro, has a similar effect in the left hemisphere but causes a transient period of memory loss from 10-20 min post-training when injected in the right hemisphere. CyA (5 nM and 20 nM), which inhibits both PP2B and PPIase activity, causes permanent memory loss from 60 min post-training when injected into the left hemisphere, an effect also observed following administration of FK506 (20 nM), which also inhibits PP2B and PPIase activity, and [MeVal(4)]CyA (5 nM), which inhibits PPIase activity but not PP2B activity. Administration of CyA (20 nM) and FK506, but not [MeVal(4)]CyA, in the right hemisphere leads to a transient period of memory loss from 10-20 min post-training. These results confirm significant roles for phosphatases and PPIases in memory processing but challenge previous conclusions drawn on the basis of bilateral drug administration protocols.