Associative memoryofthemollusc Hermis- senda crassicornis, previously correlated withchanges ofspe- cific K+ currents, protein phosphorylation, andincreased synthesis ofmRNA andspecific proteins, ishereshowntobe accompanied bymacroscopic alteration inthestructure ofa single identified neuron, themedial typeBphotoreceptor cell. Fourtofive daysafter training, terminal arborizations ofB cells iontophoretically injected withNi2+ ions andthentreated withrubeanic acid weremeasured withcharge-coupled device (CCD)-digitized pseudocolor images ofoptical sections under "blind" conditions. Boundary volumes enclosing medial-type B-cell arborizations fromclassically conditioned animals were unequivocally reduced compared withvolumes fornaive ani- malsorthose trained withunpaired stimuli. Branch volume magnitude wascorrelated withinput resistance ofthemedial typeB-cell soma.Suchassociative learning-induced structural changes mayshare function with"synapse elimination" de- scribed indevelopmental contexts. Associative memoryofthemollusc Hermissenda crassicor- nis-i.e., aremembered link between atleast twodiscrete stimuli previously associated intime-has beenshowntobe accompanied bychanges ofmembrane currents (1-4), changes inprotein phosphorylation (5), andchanges insyn- thesis ofmRNA andofspecific proteins (6, 7). These changes havebeenmeasured either insingle identified neurons orin their immediate vicinity andcanaccount forstorage and recall ofaPavlovian conditioned response bytheintact animal. Morphological changes ofneurons, unlike those changes just mentioned, havenever beendirectly related to associative memoryofHermissenda oranyother species, although theyhavebeenobserved tooccur inavariety of developmental (8-13) andtraining (14-18) contexts. Anin- herent difficulty withthetraining experiments istodistin- guish thosetraining-induced modifications duetomotor activity andsensory stimulation themselves fromthose that store memoryforlater recall. Suchadistinction ispossible withanassociative paradigm that includes acontrol group receiving sensory stimuli notassociated intimebutwith intensity andduration identical tothose usedtoassociatively train theanimals. Herewe report markedanatomic changes ina single identified neuron (the medial typeBcell) ofHermissenda in animals (paired group, n = 8)that hadbeenassociatively trained withpaired visual andvestibular stimuli. These changes weremeasured incomparison withneurons from animals receiving either notraining stimuli (naive group, n= 8)ortoneurons fromanimals receiving stimuli that were explicitly unpaired atrandomly varying temporal intervals (unpaired group, n= 8). Because there werenodifferences between typeBcells fromnaive animals compared withcells fromunpaired animals, theanatomic changes described belowforthepaired groupwerespecific tothetemporal association ofthetraining stimuli and, therefore, arenotdue totheeffect ofsensory stimulation itself andaccompanying nonassociative behavioral activity. Fourdaysafter thelast of4daysoftraining,t "blind" time-lapse video-based analysis demonstrated thepreviously observed acquisition ofaclassically conditioned response: light-elicited contraction oftheanimal's foot. Analysis of variance (ANOVA)andScheffd contrasts between groups demonstrated that thepaired group clearly differed (P<0.02) infootcontraction fromtheunpaired andnaive groups, which werenotdifferent fromeachother (Fig. lA,Table 1). Allcellular observations weremadeonasingle identified neuron, themedial typeBphotoreceptor, forwhich previous studies haveshown(1-4) particular K+ currents (IAand ICa2+-K+) aremodified fordays after classically conditioned behavior isacquired. Thiscell could beidentified ineach animal byits location within theHermissenda eyeaswell as bynumerous electrophysiological