The functional activity of the luteinizing hormone (LH) receptor can be regulated not only by gonadotropins, but also by its low molecular weight agonists, which, in contrast to gonadotropins, bind to the allosteric site located in the transmembrane channel of the receptor. The most promising low molecular weight agonists are thienopyrimidine derivatives, which are structural analogs of the Org 43553 compound. The purpose of this work was to synthesize novel thienopyrimidine derivatives—5-amino-N-(tert-butyl)-4-(3-(2-methoxynicotinamido)-phenyl)-2-(methylthio)thieno[2,3-d]pyrimidine-6-carboxamide (TP-21), 4-((3-(5-amino-6-(tert-butylcarbamoyl)-2-(methylthio)thieno[2,3-d]pyrimidine-4-yl)phenyl)carbamoyl)pyridine 1-oxide (TP-22), and 5-amino-N-(tert-butyl)-4-(3-(2-chloroxynicotinamido)phenyl)-2-(methylthio)thieno[2,3-d]pyrimidine-6-carboxamide (TP-23)—and to investigate their effects on adenylate cyclase (AC) activity in rat testicular membranes in vitro and on testosterone levels in male rats following intratesticular or intraperitoneal administration in vivo. Compounds TP-21, TP-22, and TP-23 stimulated basal AC activity in rat testicular membranes with EC₅₀ values of 1556, 358, and 372 nM; their efficiency was ordered as follows: TP-23 > TP-21 ≈ TP-22. When thienopyrimidines (10^–4 M) were applied in combination with human chorionic gonadotropin (HCG, 10^–8 M), the AC-stimulating effect of HCG was maintained, and, at an HCG concentration of 10^–10 M, the effects of thienopyrimidines and HCG on AC activity were additive. Intratesticular administration of 10 mg/kg TP-21, TP-22, and TP-23 increased testosterone levels in male rats: in 5 h after treatment, its levels were 32.8, 36.4, and 76.9 nM, respectively, higher than in the control group. Following intraperitoneal administration, TP-21 and TP-22 had little effect on testosterone levels, while TP-23 induced a significant increase in testosterone levels (by 34.8 and 18.9 nM in comparison to control in 1 and 3 h, respectively). These data suggest that compound TP-23 is an active stimulator of testosterone synthesis and secretion and represents a promising basis for development of highly efficient LH receptor agonists.