The mechanical responses to potassium of the rabbit and rat uterus were different. The force of a potassium-induced contracture showed a rapid decline in the rabbit uterus, whereas it was fully maintained for a long period in the rat uterus. The increase in 45Ca influx following exposure to high KCl was substantial and similar (0.3 mmol kg-1) in both rat and rabbit uterus. Calcium uptake in post nuclear supernatant (PNS) from the rat myometrium was significantly lower than that from the rabbit myometrium in both the rate and capacity of uptake. This uptake was reduced to about 10% in the presence of sodium azide in PNS from both rat and rabbit myometria. Both the rate and capacity of calcium uptake by mitochondria isolated from PNS of rat myometrium were considerably lower than that by rabbit myometrial mitochondria. Microsomes isolated from rat and rabbit PNS had very low capacity for calcium uptake compared with mitochondria. No difference was found in the rate or the capacity of uptake between the rat and the rabbit myometrial microsomes. The data support the argument for a role of mitochondria in myometrial relaxation and further provide evidence for a considerable species difference in calcium uptake by myometrial mitochondria.