The present study was carried out to determine if augmentation of phrenic nerve activity during the crossed phrenic phenomenon temporally coincides with the morphological changes in the phrenic nucleus that we have observed in previous studies. This investigation consisted of two experiments in spinal cord hemisected young adult female Sprague-Dawley rats. Crossed phrenic activity was quantitatively assessed from the left phrenic nerve after bilateral vagotomy and sectioning of the right phrenic and accessory phrenic nerves. The first experiment involved serial recordings of crossed phrenic activity performed on each of 4 animals at hourly intervals ranging from 1 to 6 h after spinal cord hemisection. The second experiment consisted of single recordings from each of 24 animals at one of the following time intervals after hemisection: 1/2, 1, 2, 4, 12, and 24 h. Recording conditions were standardized at each recording session in both experiments by paralyzing the animals, regulating temperature and blood pressure, and controlling end tidal PCO2 with a volume ventilator. Crossed phrenic activity was induced by stopping the ventilator and quantitated by measuring the area under the integrated waveform of the largest respiratory burst. The results revealed a small, statistically insignificant increase in crossed phrenic activity at 1 h compared to the 30-min recordings. At 2 h there was a large, statistically significant increase in activity. Experiment one showed further increases from 3 to 6 h. The second experiment showed a smaller increase from 2 to 4 h and then maintained this level at 12 and 24 h.(ABSTRACT TRUNCATED AT 250 WORDS)