Treatment of GH₃ pituitary cells with p-chloromercury-benzenesulphonate (PCMBS) increased the cytosolic Ca²⁺ concentration ([Ca²⁺]_i). This effect was reversed by dithiothreitol and blocked by L-type Ca²⁺ channel antagonists or by Na⁺ removal. PCMBS increased membrane conductance and depolarized the plasma membrane. Apart from minor effects on K⁺ and Ca²⁺ channels, PCMBS increased (6 times at -80 mV) an inward Na+ current whose properties were similar to those of a background Na⁺ conductance (BNC) described previously, necessary for generation of spontaneous electrical activity. In rat lactotropes and somatotropes in primary culture PCMBS also produced a Na⁺-dependent [Ca²⁺]_i increase, whereas little or no effect was observed in thyrotropes, corticotropes and gonadotropes. The Na⁺ conductance elicited by PCMBS in somatotropes seemed to be the same as the one stimulated by the hypothalamic growth hormone (GH) releasing hormone, which regulates membrane excitability and GH secretion. The BNC studied here could play a physiological role regulating excitability and spontaneous activity and explains satisfactorily the [Ca²⁺]_i-increasing actions of the mercurials reported previously in several excitable tissues.