We have examined the effects of low Ca2+ solutions, Co2+, and ryanodine on the isometric tension and contraction speed of isolated, developing mouse EDL muscles. Twitch responses of young muscles (7-14 days postnatal) were more sensitive to lowered [Ca2+]o than those of more fully developed muscles (22-35 days postnatal). Responses of EDL muscles from a middle-aged group (15-21 days postnatal) were intermediate between the two other groups. Overall, the time course of contraction in a single twitch was accelerated by low [Ca2+]o. Ca(2+)-free solution induced a 7.95 and 9.25 mV depolarization in young and "old" muscle fibres, respectively. The presence of cobalt ions (5 mM) in the Krebs solution had a similar effect as Ca(2+)-free Krebs in terms of reduction of the isometric twitch and tetanic tensions of EDL muscles from the various age groups. In contrast, the shortening of the contraction time seen with Ca(2+)-free solution did not take place following exposure to Co(2+)-containing solutions. Finally, young (7-14 days postnatal) muscles were less sensitive to the inhibitory action of ryanodine on the twitch compared with more fully developed muscles (22-35 days postnatal). Taken together, our results indicate that from birth to maturity, there is a gradual change in the spectrum of calcium utilization for the contractile process.