The first three ships finished, Pará, Alagoas and Rio Grande, participated in the Passage of Humaitá in February 1868.

The ships were split between the newly formed Upper Uruguay (Portuguese: Alto Uruguai) and Mato Grosso Flotillas after the war.

The Pará-class river monitors were designed to meet the need of the Brazilian Navy for small, shallow-draft armored ships capable of withstanding heavy fire during the Paraguayan War, which saw Argentina and Brazil allied against Paraguay.

The monitor configuration was chosen as a turreted design did not have the same problems engaging enemy ships and fortifications as did the casemate ironclads already in Brazilian service.

The engines produced a total of 180 indicated horsepower (130 kW) which gave the monitors a maximum speed of 8 knots (15 km/h; 9.2 mph) in calm waters.

[5] Most unusually the guns' Brazilian-designed iron carriage was designed to pivot vertically at the muzzle; this was done to minimize the size of the gunport through which splinters and shells could enter.

For the engagement the three river monitors were lashed to the larger ironclads in case any engines were disabled by the Paraguayan guns.

Alagoas was under repair at São José do Cerrito until mid-March, although Pará joined a squadron to capture the town of Laureles on 27 February.

On 23 March Rio Grande and Barroso sank the Parguayan steamer Igurey and both ships were boarded by Paraguayan soldiers on the evening of 9 July, although they managed to repel the boarders.