The possibility of reversing the hypoxanthine induced 2-cell block in mouse embryos when cultured in conditions supplemented with compounds that increase (FSH, hMG, IBMX, hCG) or inhibit (GnRH-analogue) cAMP was assessed. When embryos were cultured in Ham's F-10 without hypoxanthine supplemented with each of the above compounds, no inhibition of blastocyst development was observed. Embryos were then cultured in Ham's F-10 with hypoxanthine supplemented again with each compound. For the addition of GnRH-analogue or FSH, the rate of blastocyst formation was comparable with that of the control medium with hypoxanthine alone. Instead, the addition of IBMX or hMG reversed the induced block. There was no reversible effect for the addition of 2 micrograms/ml hCG while the latter was observed with higher doses. The results from GnRH-analogue and IBMX addition show that, contrary to what was found for oocytes, stimulation of cAMP reverses the hypoxanthine-induced block in mouse embryos. FSH and hCG also had effects opposite to those observed for oocytes. It is unknown why hMG (FSH + LH) reverses the block. A lower cAMP degradation rate resulting in a higher cAMP level is a possible explanation. Our results provide further evidence that cleavage arrest by hypoxanthine has a different mechanism than the hypoxanthine-induced arrest of meiosis.