1. The free cytoplasmic Ca2+ concentration [( Ca2+]i) was measured in the tonic rabbit pulmonary artery and the phasic ileum smooth muscle. 2. Force development and [Ca2+]i were determined during either cumulative or non-cumulative additions of [Ca2+]o to smooth muscles depolarized with 140 mM-K+ solutions. 3. The level to which [Ca2+]i declined in Ca2+-free, 140 mM-K+ solutions was significantly lower in the ileum (40 +/- 4 nM) than in pulmonary artery (77 +/- 5 nM) smooth muscle. 4. The level of [Ca2+]i reached during non-cumulative superfusion with 10 microM and 1 mM [Ca2+]o was higher in the pulmonary artery than in the ileum. 5. The force level reached for a given [Ca2+]i was also higher in the pulmonary artery than in the ileum. 6. During maintained depolarization there was a marked decrease in the sensitivity of ileum smooth muscle tension to [Ca2+]i. 7. We conclude that significant differences exist in the Ca2+ sensitivity of the regulatory/contractile apparatus among different smooth muscles; the lower sensitivity of depolarized ileum than pulmonary artery to [Ca2+]o is due to both differences in Ca2+ metabolism and in the Ca2+ sensitivity of the regulatory contractile system. We suggest that these two mechanisms also contribute to the decline in force during a phasic K+ contracture, and that desensitization to [Ca2+]i contributes to the decline of the K+ contracture in the ileum.