The rapid spread of interest and experimental work in early experience has led to confused and contradictory claims. This thesis reviews the literature to assess these claims and investigates certain hypotheses. Experiments were designed to test (1) the effects of neonatal stimulation, (2) the effects of pre v post-weaning environments, and (3) the effects of handling in the golden hamster. This experimental animal was used to determine cross-species applicability of infantile stimulation theory. Experimental work consisted of submitting litters to early "burrow" or laboratory environments preweaning, and to laboratory or free-enriched environments post weaning. Other litters were subjected to various handling schedules - days 1-21, days 5-21 and "non-handled" controls in order to understand the contradictory results reported. Dependent variables included, physiologically, growth observation, brain and adrenal weight analysis, and plasma cortisol assay; and, behaviourally, open-field behaviour, response to novel objects and discrimination learning ability: all variables typically used in this field. The data were subjected to analysis of variance. Results show early stimulation in the golden hamster to have none of the "beneficial" effects found in the rat; rather it leads to increased emotionality, neophobia, impaired learning and poor physical development. These effects cannot solely be attributed to impaired hypothalamic maturation nor alteration in maternal care, as demonstrated by the handling conditions. Both early "burrow" environments and later free-enriched experience effect clear adaptation in the subjects with decreased emotionality, faster reactivity and good learning ability; the early environment affected physiological changes within the animal, whilst the enrichment produced its effect via experiential factors. Wider usage of species, taking into account their known natural history, and a more ethological experimental approach appears necessary to gain a true understanding of mediators and their consequences in this field. No extrapolation to higher organisms is valid at this stage.