We previously demonstrated that early isolation has profound effects on the morphology of the dentate granule cells and field CA3 pyramidal neurons. Aim of the present study was to analyze the effects of early environment on the morphology of field CA1 pyramidal neurons, the third element of the hippocampal trisynaptic circuit. The dendritic trees and the soma of field CA1 pyramidal neurons were quantified in Golgi-stained brains of guinea-pigs of both sexes raised in either a social or an isolated environment. Based on the different pattern of the apical dendritic tree two major classes of CA1 pyramidal neurons were recognized (monotufted neurons and bitufted neurons). In males isolation induced in both neuron types a decrease in the number of low order apical branches but in the apical tree of the monotufted neurons isolation induced an increase in the number of intermediate order branches and dendritic length. In isolated females the apical tree of the monotufted neurons showed a very scarce atrophy. In contrast, the apical tree of the bitufted neurons from isolated females showed a decrease in the number of low and intermediate order branches and dendritic length. In isolated males the basal tree of the bitufted neurons had a large decrease in the total number of branches and dendritic length. In contrast, in isolated females the basal tree of both neuron types showed an increase in the number of low order branches. In males but not in females isolation caused a reduction in the soma dimensions of both neuron types. No isolation-induced changes were observed in dendritic spine density in either the apical or basal dendrites. The results demonstrate remarkable structural changes in CA1 pyramidal neurons following early isolation and a different reactivity to environment of the two CA1 pyramidal neuron types, their apical and basal trees and the two sexes. The neuroanatomical changes caused by isolation in field CA1 and in the two other elements of the trisynaptic circuit are likely to be associated with changes in the physiology of the hippocampal formation and in cognitive processes such as learning and memory in which the hippocampal formation plays a pivotal role.