Effect of pressure and inert gases on the twitch tension (Tmax) was measured on electrically stimulated and spontaneously beating rat atria. In stimulated preparations, pressurization to 10 MPa increased Tmax by 20-60% depending on the stimulating frequency (60-240 beats/min). The introduction of 5 MPa N2 or 5 MPa H2 at 10 MPa hydrostatic pressure decreased the Tmax by 17 +/- 6% and 13 +/- 6%, respectively. Gas effect did not depend on the stimulating frequency. Nitrous oxide (0.15 and 0.45 MPa) decreased Tmax both at "surface" and at 10 MPa. Nitrous oxide effect was slightly potentiated at pressure. In spontaneously beating preparations, compression to 10 and 15 MPa decreased beating frequency (BF) by 24 +/- 10% and 31 +/- 8% and increased Tmax by 60 +/- 35% and 105 +/- 33%, respectively. The tension increase is partly due to the direct pressure effect and partly due to the negative force-frequency relation in the rat atria. Introduction of inert gas increased BF and decreased Tmax. The potency of the gases was in the same order for both variables: He less than H2 less than N2.