Histone deacetylase inhibitors are a new and promising drug family with a strong anticancer activity and potent modulation of connexin expression. The restoration of connexins in cancer cells has been suggested as a possible mechanism to control tumor progression. The aim of this study was to investigate the effects of 4-phenylbutyrate (4-PB) on the growth of human pancreatic cell lines in vitro and in vivo with a focus on connexin modulation. The proliferation of tumor cells was determined using an MTT assay, and the effect of 4-PB in vivo was studied in a chimeric mouse model. The expression and localization of connexin 43 (Cx43) were assessed by Western blot, immunofluorescence microscopy, and immunohistochemistry. Antitumoral activity was assessed by immunohistochemistry for Ki-67 and histone H4. Treatment with 4-PB resulted in the significant in vitro and in vivo growth inhibition of pancreatic tumor cells. The reduction of the xenograft tumor volume was associated with the inhibition of histone deacetylation and decrease in cell proliferation. Treatment with 4-PB caused a significant increase in the Cx43 expression in T3M4 cells (up to 2.8-fold). The newly synthesized Cx43 was localized in the cytoplasm but not on the cell membrane. Treatment with 4-PB inhibited the proliferation of human pancreatic tumor cells in vitro and in vivo and increased the expression of Cx43. Therefore, 4-PB might be useful in the therapy of pancreatic cancer.