Intrauterine respiratory movements in the fetal guinea pig were detected from about 0.6 of term by the use of chronically implanted tracheal and amniotic catheters. Three distinct patterns were seen: 1) "slow" breathing recurring at a rate of 1-8/min; 2) "rapid irregular" breathing in which episodes last a few seconds to 40 min and the frequency of breathing is high (greater than 0.5 HZ) (the respiratory movements are associated with negative tracheal pressures of of 0.5-20 Torr); and 3) "rapid regular" breathing which recurs in bursts lasting 0.6-6.8 min with a maximum incidence of 3/h. Inspiratory efforts in the range of 10-20 Torr were uniform during a single episode. All three types of respiratory activity were continued into first stage labor, both during and between contractions, but were absent during second stage labor.