Structure-affinity relationship (SAR) studies for the 5-HT1A receptor site are presented for two series of heterobicyclic phenylpiperazines with N4-aralkyl substituents: 4-aralkyl derivatives of 1-(2,3-dihydro-1,4-benzodioxin-5-yl)piperazine (3) and 1-(benzo[b]furan-7-yl)piperazine (4). Their affinities for 5-HT1A receptors range from 0.15 to 28 nM and thus emphasize the importance of N4-substitution. By combining the SAR of these N4-aralkyl series with the recently published SAR of the N4-alkyl-substituted phenylpiperazines, the nature of the interaction of the N4-substituted phenylpiperazines and the 5-HT1A receptor was further examined using comparative molecular field analysis (CoMFA). To discriminate between two postulated hypotheses, CoMFA models were built and validated utilizing cross-validation, bootstrapping, and randomizing techniques. The model based on a N4-substituent alignment in which all N4-substituents are equally oriented in space was selected for further evaluation. According to the CoMFA/PLS analysis, the steric and electrostatic field properties contribute in a 98:2 ratio to the affinity found for the 5-HT1A receptor. Increasing steric bulk was found to be positively as well as negatively related to affinity depending on the distance of the bulk's center from the N4-nitrogen. The location of these steric CoMFA contour levels are well defined in space when the defined alignment rules are followed. Because CoMFA does not take hydrogen bonding into account, this could indicate that the contribution of the amide function (its ability to interact through hydrogen bonding), as present in the N4-substituents, to affinity is of minor importance.