The effects of an LHRH agonist on LH- and forskolin-stimulated cyclic AMP and testosterone production have been investigated in purified rat Leydig cells in vitro. In agreement with previous results it was found that preincubation with LHRH agonist inhibited subsequent LH-stimulated cyclic AMP production. At least 2 h preincubation was required and this effect of the LHRH agonist was negated by the protein synthesis inhibitor cycloheximide and by the phosphodiesterase inhibitor methylisobutylxanthine (MIX). Forskolin-stimulated cyclic AMP production was not inhibited by the LHRH agonist. Forskolin increased testosterone production to the same levels attained by LH and preincubation with LHRH agonist increased both forskolin- and LH-stimulated testosterone production. The data obtained suggest that LHRH agonist increases the synthesis of an inactive form of phosphodiesterase (or associated protein) which is activated by LH via a mechanism not involving cyclic AMP.