Thyroid hormone (TH) was shown to induce actin cytoskeleton polymerization in hypothyroid astrocytes and osteoblastic cells by a non-genomic mechanism. Polyadenylation of GH mRNA, a process that depends on cytoskeleton-associated proteins, was also shown to be regulated by TH. Here we investigated by histochemistry and immunohistochemistry whether acute (100 µg/100 g BW, iv, for 30 min) or chronic (5 µg/100 g BW, ip, 5 days) administration of T₃ to thyroidectomized (Tx) and sham-operated (SO) rats, affects the somatotrophs F-actin cytoskeleton arrangement and its potential repercussion on GH synthesis and secretion. Thyroidectomy dramatically decreased the amount of somatotrophs F-actin content and induced the disassembly of the actin cytoskeleton. These effects were reversed by acute and chronic administration of T₃. In addition, in Tx rat somatotrophs, GH labeling was detected mostly at the cell periphery. After 30 min of T₃ administration GH labeling decreased at periphery and increased in the perinuclear region, suggesting that GH secretion and synthesis were stimulated by T₃. No differences were detected in the total actin protein content, although a decrease in the F- and increase in G- actin contents were detected in Tx rats pituitaries, a panorama that was reversed by acute T₃ treatment, as shown by western blotting analysis. The SO animals' somatotrophs were only mildly affected by acute T₃ administration. The results indicate that the T₃-induced rapid alterations on somatotrophs actin cytoskeleton and GH cellular distribution resulted from actin filaments rearrangement, which characterizes a non-genomic action.