Studies involving the role of ghrelin (GHREL) in regulating the proliferative activity of various cell types have obtained variable results depending primarily on the experimental model applied. It was recently reported that neither GHREL nor obestatin (OBS) affected the proliferative activity of cultured rat adrenocortical cells. In view of the conflicting results, we investigated the effects of GHREL and OBS on the proliferative activity of rat adrenocortical cells in a model of bilateral enucleation-induced adrenocortical regeneration in the rat. Rats were sacrificed 5 or 8 days after surgery. Twenty-four hours before being sacrificed, the appropriate groups were infused with 3 nmol GHREL or OBS/100 g. The mitotic index was assessed using the stachmokinetic method with vincristine. In comparison with intact rats, expression levels of ppGHREL, BAX, JUN-B and JUN-C genes were notably higher in regenerating adrenals, and neither GHREL nor OBS infusion affected these levels. Expression levels of the GHS-R, GPR39v2 and FOS genes were affected neither by adrenal enucleation nor GHREL or OBS infusion. Expression of only two studied genes, GPR39v1 and EGR1, was regulated by OBS. In the regenerating adrenal glands, GPR39v1 and EGR1 mRNA levels were higher than the levels in intact animals. GHREL infusion had no effect while OBS infusion notably stimulated GPR39v1 mRNA levels in the regenerating adrenal gland and evoked an opposite effect on EGR1 mRNA. OBS administration resulted in a potent decrease in the mitotic index of the studied cells, an effect found at both days 5 and 8 of the experiment. GHREL exerted a similar effect only at day 5 of adrenocortical regeneration. Neither GHREL nor OBS had an effect on blood aldosterone concentrations. GHREL infusion lowered plasma corticosterone concentration at day 5 but not 8 of the experiment, while OBS administration was ineffective. Thus, this study is the first to demonstrate that, in vivo, both GHREL and OBS inhibit the growth of the regenerating adrenal cortex. Moreover, the data suggest that the effect of OBS might be, at least in part, mediated by the EGR1 pathway known to be critical in cell proliferation.