The effect of route of administration on induction of unscheduled DNA synthesis (UDS) in mouse germ cells in vivo was studied using two germ cell mutagens, methyl methanesulfonate (MMS) and triethylenemelamine (TEM). The chemicals were administered to male mice (C3Hf X 101)F1 by IP injection or gavage using acute or 5-day subacute regimens. After completion of dosing, methyl-[3H]thymidine [( 3H]TdR) was injected into the testes, and spermatozoa were collected 16 days later. The sperm heads were isolated, and UDS was determined by the amount of [3H]TdR incorporated. Acute administration of MMS (2-100 mg/kg) induced a strong, dose-related UDS response. The response was slightly higher with IP injection than with gavage. The UDS response after five daily doses of 50 mg MMS/kg was 20-30% higher than that induced by a single IP or gavage dose. Acute administration of TEM (0.05-4.0 mg/kg) by IP injection or gavage induced weak and variable responses. Retesting TEM using inbred C3Hf mice produced weak but exposure-related responses with both acute IP and gavage treatments. There was a slight increase in UDS response with subacute IP injection but not with subacute gavage. Acute testicular injection of TEM produced a higher but more variable UDS response. The study showed that gavage, as well as IP injection, can be used for the administration of test chemicals and that the subacute 5-day regimen induced a higher UDS response than the acute regimen. Furthermore, the testicular route may enhance the detection of weak UDS inducers.