An activity-responsive implantable medical device comprising: a connector connectable to an implantable medical lead (20,30) having at least one pacing electrode (22, 24, 32); a pulse generator (130, 135) connected to said connector and configured to generate pacing pulses to be applied to said at least one pacing electrode (22, 24, 32); an activity processor configured to i) receive a sensor signal representative of an activity level of a subject from an activity sensor, ii) determine a current activity level of said subject based on said sensor signal and iii) determine a time period during which said current activity level determined based on said sensor signal has exceeded a first threshold level; and a pulse generator controller connected to said pulse generator (130, 135) and configured to control said pulse generator (130, 135) to generate said pacing pulses, characterized in that: said activity processor is configured to compare said current activity level determined based on said sensor signal with said first threshold level; said pulse generator controller is configured to control said pulse generator (130, 135) to switch from generating pacing pulses at a predefined base pacing rate as long as said current activity level as determined by said activity processor based on said sensor signal is equal to or below a second threshold level that is lower than said first threshold level to a predefined activity pacing rate when said current activity level determined by said activity processor based on said sensor signal exceeds said second threshold level, said predefined activity pacing rate is higher than said predefined base pacing rate; and said pulse generator controller is configured to control said pulse generator (130, 135) to increase the pacing rate of generating pacing pulses from said predefined activity pacing rate based on a length of said time period. a connector connectable to an implantable medical lead (20,30) having at least one pacing electrode (22, 24, 32); a pulse generator (130, 135) connected to said connector and configured to generate pacing pulses to be applied to said at least one pacing electrode (22, 24, 32); an activity processor configured to i) receive a sensor signal representative of an activity level of a subject from an activity sensor, ii) determine a current activity level of said subject based on said sensor signal and iii) determine a time period during which said current activity level determined based on said sensor signal has exceeded a first threshold level; and a pulse generator controller connected to said pulse generator (130, 135) and configured to control said pulse generator (130, 135) to generate said pacing pulses, characterized in that: said activity processor is configured to compare said current activity level determined based on said sensor signal with said first threshold level; said pulse generator controller is configured to control said pulse generator (130, 135) to switch from generating pacing pulses at a predefined base pacing rate as long as said current activity level as determined by said activity processor based on said sensor signal is equal to or below a second threshold level that is lower than said first threshold level to a predefined activity pacing rate when said current activity level determined by said activity processor based on said sensor signal exceeds said second threshold level, said predefined activity pacing rate is higher than said predefined base pacing rate; and said pulse generator controller is configured to control said pulse generator (130, 135) to increase the pacing rate of generating pacing pulses from said predefined activity pacing rate based on a length of said time period. said activity processor is configured to compare said current activity level determined based on said sensor signal with said first threshold level; said pulse generator controller is configured to control said pulse generator (130, 135) to switch from generating pacing pulses at a predefined base pacing rate as long as said current activity level as determined by said activity processor based on said sensor signal is equal to or below a second threshold level that is lower than said first threshold level to a predefined activity pacing rate when said current activity level determined by said activity processor based on said sensor signal exceeds said second threshold level, said predefined activity pacing rate is higher than said predefined base pacing rate; and said pulse generator controller is configured to control said pulse generator (130, 135) to increase the pacing rate of generating pacing pulses from said predefined activity pacing rate based on a length of said time period.