Profound somatotroph hypoplasia in the dwarf (dw/dw) rat is accompanied by an estrogen-dependent induction of prolactin (PRL) secretion by the growth hormone secretagogue, GHRP-6. Using electron microscopy, we have demonstrated that the reduction in the somatotroph population in the dw/dw pituitary is accompanied by the presence of a morphologically distinct lactotroph subpopulation. In these cells, which did not co-express growth hormone, the size, shape and number of the secretory granules was between that of the type I and type II lactotrophs. We have therefore called these cells, ‘intermediate’ lactotrophs. The ‘intermediate’ lactotrophs accounted for up to 30% of the total PRL-positive cell population in dw/dw males, and up to 12% in females. Using tannic acid to quantify the fusion of secretory granules, we have shown that the ‘intermediate’ lactotrophs are unresponsive to either GH-releasing factor (GRF), or TSH-releasing hormone, but exhibit a sexually dimorphic secretory response to acute ghrelin treatment, granular fusions being 4-fold higher in females. No cell matching the morphology of the novel lactotroph subpopulation was observed in the pituitary of the GRF-insensitive lit/lit mouse. However, ablation of GRF neurons with neonatal MSG treatment had no effect on the population of ‘intermediate’ lactotrophs in the dw/dw rat. Thus, the presence of the ‘intermediate’ lactotrophs in the dw/dw pituitary appears to be independent of the function of the GRF neurons.