1. Resting membrane potential (Vmp), input resistance (Rn), rheobase (Irh), and after hyperpolarization duration (AHPdur) and amplitude (AHPamp) were measured in 38 phrenic motoneurons of anesthetized, paralyzed, and artificially ventilated cats during hypocapnic apnea. The mean +/- SD and range of values for these variables were as follows: Vmp, -68 +/- 5mV (range: -60 to -82); Rn, 1.3 +/- 0.6 M omega (0.6-2.4); Irh, 9.7 +/- 5 nA (2-20); AHPdur, 68 +/- 19 ms (37-134); AHPamp, 3.3 +/- 1.8 mV (1.0-8.5). In 31 motoneurons, the membrane potential level at which firing occurred (Vthr) during intracellular current injection was measured. The mean value of Vthr was -58 +/- 3 mV (range: -52 to -64). 2. A histogram of Rn revealed a bimodal distribution. Also a plot of Irh against Rn showed a grouping of the motoneurons into two subpopulations: 1) low-Rn and high-Irh cells, called type L neurons, and 2) high-Rn, low-Irh cells, called type H neurons. The overall negative linear correlation between Irh and Rn (r = -0.85; P less than 0.0001) resulted from this grouping rather than from a strictly linear relation between these two variables. 3. Electrical properties were compared for type L (n = 20) and type H (n = 18) phrenic motoneurons. The following mean values were found for each group, respectively: Rn, 0.8 and 1.8 M omega; Irh, 13.7 and 5.3 nA; AHPdur, 58 and 79 ms; AHPamp, 2.4 and 4.4 mV. All differences were significant (t test, P less than 0.001). Mean Vthr was the same for the two groups. 4. Comparison of these data with those available for lumbosacral motoneurons revealed that almost all investigated electrical properties of type L and type H phrenic motoneurons are similar to the analogous properties of type F (fast twitch) and type S (slow twitch) lumbosacral motoneurons, respectively. The apparent exception is the lower mean value of Irh for type L phrenic motoneurons compared with type F lumbosacral motoneurons. 5. For 13 cells, membrane potential was continuously monitored while spontaneous respiratory activity was restored by increasing CO2. It was found that at approximately the same end-tidal CO2 (about 7%) and a similar end-expiratory mean membrane potential level (approximately -70 mV), mean amplitude of peak inspiratory synaptic depolarization was higher in type H motoneurons (8.8 mV, n = 5) than in type L (2.9 mV, n = 8; P less than 0.001).(ABSTRACT TRUNCATED AT 400 WORDS)